Can't tell about M20 vs M30 but there is a difference between M-line models and PB-line models. PB-line are/were made personally by Bernabe (Sr or Jr or both) as opposed to M-line models that are/were made by other luthiers/workshops to Bernabe specs. So if your budget allows then it's better to buy a PB line model with the "PB" spamp on the label. A used PB-line guitar can be comparable in price to a new M-line one. India model is the cheapest in the PB-line family.
On the top finish topic: I refinished my Takamine C136S from original lacquer to French, the difference in sound was quite noticeable and the tone improved significantly. So yes, French polish makes a difference.
Paulino Bernabe 'India' 2001
Takamine C136S 1976
Masaru Kohno No.6 1967