Old thread but important topic. I chime in two years late.
The low bass strings on an extended range guitar are typically fatter than the usual low E2 string.
A little more space between them is nice, also because in reaching to the bass side of the fingerboard,
the fingers come down at more of an angle, so it is harder to play cleanly on two adjacent strings in the
bass, with the fatter strings. However, I myself have not yet experimented with a graduated string spacing,
and if anyone has done so, it would be interesting to hear a report.
It is also a question of the string spacing at the nut (center to center in mm) which you are already used to playing on.
If this spacing is different on your 8-string than it was on your 6-string, it will be an added difficulty in
adapting to the new instrument. String spacings at the nut range from a very narrow 7mm or less used
on some electric guitars, to about 7.5 or 8mm for some "classic crossovers" built for steel string players, to about
8.5mm for many classical guitars. My own preference is for 9mm, which allows me to play more cleanly
without the sides of my fingers damping adjacent strings; this is good for folks with fat fingers.
The individual string spacing is far more important than the total width of the neck. I also was afraid of the total width
of the neck when I had one of my first ERGs built, but I made an expensive mistake when I had the neck made too narrow,
because it made it very difficult to play after playing on a wider string spacing. This was such an important lesson for me
that I must emphasize it.
While you are at it, consider the distance from the outside strings to the edge of the fingerboard. Many guitars only have about
2mm here, which allows the string to slip off the fingerboard when trying to reach difficult chords. My preference is for a full
4mm on each side. To reach the extreme treble and bass strings for chords spanning a 22nd (routinely possible on many 8-strings,
depending on your tuning) involves a stretch which you won't be used to yet, and it's nice to have that extra security so as
not to push the string over the edge of the fingerboard.
So: my formula for the neck width at the nut:
SS = desired spacing between strings center to center in mm
SS x number-of-strings-minus-1, plus 8mm for the edge spacing = total width of FB at nut.
By the way, I use an 11mm spacing at the bridge.
I'm playing nine strings and am very happy. My 6-string and uke are wall decorations.