Hi, this is the original poster chiming back in.
I have been obsessed with this guitar for an entire year, making first drawings last January, received it end of August and have spent since then learning to play it. (Aside from learning the expanded fingerboard layout, the major obstacle was having set the strings so close together, requiring a tightening up of my technique with both hands, changing from my previous work ax with a 9 mm spacing to this with 7.6 mm spacing. However, this obstacle is overcome.)
My big question was always about the practicality of the high A string. Many comments from 8 string players were duly noted, and I hoped to bypass the issues they spoke of by making the high A short, 56 cm. I went from a FC .47mm string first to .435 mm and then to a hair-thin .405 for a while. While the guitar sounded pretty nice, it was always a little stiff sounding all the way around, as well as the high A sounding a little thin. I was regarding it as my experimental thing.
SO!!! After listening for hours to Nigel North's recordings of Dowland, I tuned it down a step at last. WHAT a difference. It's a real guitar, deep booming basses and nice singing trebles, so profoundly different (and, I might add, more like the sound that I expected from the builder since I have two other guitars of his) that I can't refuse to use this tuning instead of the higher one that I planned. Now it's: (bottom to top)
E' - A' - D - G - c - f - a - d' - g' = double bass plus Renaissance G lute tuning.
this does present me with the issue I was afraid of and which Shub mentioned as why he went back to standard tuning: it's a whole new diapason for an improvisor / arranger / composer to deal with. But I have been re-wiring my brain for two years now, first with my 7-string and now this, and I think that I am up for the ongoing re-wiring job to learn fingerboard harmony with this tuning. The sound is worth it.
I have some thoughts to share about high A strings in general. First a specific response to Shub who mentioned that he's going to try a FC .57 mm for a high A. I don't think so. I got down to .405 mm... A .57 mm makes a nice E string at 650 mm but it's pretty high tension and .52 mm is more comfortable even for a 650 mm E string.
Next a historical overview. (Parenthetically I don't buy James Tyler's strict separation of vihuela and guitar in the XVI century - this is pedantic although "literally" true - the instrument has had so many variations of form and name, from zither to ukelele etc. That was a temporary semantic distinction lasting a few short years in the 1600s. The whole lute-guitar-viol family shared tunings and techniques and was completely intertwined with many crossovers,from my point of view. )
SO: We see that Luis Milan writes his first fantasy in the Dorian mode with the high string being "La" or A. We see that Mudarra's Fantasy 10 has the high string Sol or G. We read in Galilei's "Fronimo" that he offers 12 different ways (and mentions that he has more) to map the gamut onto the lute - he's not changing the pitch, he's changing the names of the notes on the fingerboard (and he hasn't covered a complete cycle of fifths by any means). He offers a demonstrative set of ricercares in all 24 Glarean modes: 12 with G tuning, 9 with A tuning, 2 with C tuning and one with E tuning for the highest string. He is not changing the pitch, but the names of the notes. I observe that Luis Milan's music sounds beautiful and elegant with the high A tuning, but that Dowland's sounds thin unless it's in the G tuning.
At the beginning of the XVII century we see a period of chaotic experiments in different lute tunings lasting 25 years. When the dust clears, the vihuela has gone away, leaving behind the Baroque guitar which is patently just a vihuela missing its high A string (pace James Tyler.) So many vihuelas around with broken high A strings, they quit building them. In Italy the chittarone keeps the high A tuning sort of - the fingerings remain the same but the top A string is tuned down an octave and later the E string as well. Meanwhile the Baroque lute migrates in the course of the century from a G tuning to one with a high F. (The interesting thing about this tuning is that it duplicates the symmetry of the G tuning which has two sets of 3 strings with the same pattern, that is the Vielle Ton had 2 groups of 4ths separated by a third, and the later tuning has 2 groups of A - D - F.) But my point is that the high G string went down by a whole step.
So, I make it that the high A string of the vihuela was a victim of having to conform to a steadily rising pitch standard at the end of the XVI century, imposed by inflexible keyboard instruments and the need for a pitch standard for ensembles. Although Zarlino, Vicentino and Galilei were all agreed that the lute /viol /guitar family used equal temperament in the mid XVI century, and therefore could in theory map the gamut to any pitch, keyboard players used meantone for another several generations and therefore could not remap the gamut, so they drove the movement toward pitch standardization. Also, once lute-family players were required to read continuo parts in staff notation they probably had to standardize their pitch - tranposing at sight is a pain to say the least.
My current feeling is that an instrument having a high A must be a small, twangy thing closer to the uke or the mandolin. Although I haven't time and money for a thorough and scientific investigation, the experience of this year points to a high G as being a reasonable upper limit, and I want to go on record as saying so because I have been resisting this conclusion throughout the course of this thread.
I note that the Terz guitar of Giuliani was tuned to G. Also, in Mexico there are many clear remnants of 17th c. Spanish music practice, among them specifically the Mexican Vihuela which has five strings tuned A D G B E (note: vihuela with no high A), and the Mexican Requinto (a word used for a particular guitar tuning in the 16th c. - cf James Tyler) which is a little 6-string guitar which may be tuned up a fourth (A tuning) but is more commonly tuned up a third (G tuning) and which has typically a scale length of about 54 cm.
For the future, I see that an 8-string instrument tuned A - D - G - C - F - A - D - G with a modest fanned fret layout is a very practical, well sounding and conservative solution to the extended range guitar question. My next question with regard to the 9 string, is how far the low E string could be stretched out. Based on my experience of the fanned frets, which is that playing on fanned frets is far less of an issue than the mental work of mapping the extended fingerboard, I believe that a low 9th string could be 700 mm or more, maybe 720, and sound much beefier. Now I admit that it's arguable that it's not really musically necessary. But I have one now, and am finding plenty of opportunities to use those low notes, and the truth is the low E string sounds better than it did as an F sharp. I really think that the very modest fans used on the Brahms Guitar are missing an opportunity to stretch out the basses to where they would really sound like something... (my low E is 680 mm, and the total fan is 12 cm. this is twice the fan of the Brahms guitars. )
Over and Out! Happy New Year everybody and thanks so much for all your input!
- jack cat
. And I read through the whole book of two and three part Inventions & Sinfonias in the first 24 hours I had it. I've only had it a couple of weeks. You can imagine that I am both thrilled and overwhelmed. 
