May I resurrect this old thread? Sure ...
Concerning the differences between version A and B of the tanks, I did some measurements at the time when I was dealing with this issue, which I didn't report back then.
Today I ran out of gas with the low-fuel digital odometer showing 56 km.
Before swapping the tank version A with the version B + Caswell, I had consistently determined that while driving fuel-efficient, I was able to run on reserve for at least 70+ km. Once I even ran as much as 85 km before finally finding a petrol station
Anyhow ... this brings me to the measurements I did last year ...
The GT tank should hold 15 l out of factory (specs). My old tank, version A, could be filled up from fully empty, with the bike in vertical position, with 14.5 l. After Caswell-ing my second tank, version B, I did the same measurement and came up with 13.25 l. When I coated the tank, I measured the volume of Caswell I poured into the tank, then subtracted the amount that dropped out of the tank after having sloshed it around, so I know that I distributed into the tank 0.25 l of Caswell. This means that the original new-from-factory version B tank holds 13.5 l fuel ... 1 l less than version A. That correlates well with the difference in weight between the two versions. I measured roughly 1.1 kg in weight difference (version B - version A). With the density of PA being around 1.15 g/cm³ the weight difference should reduce the internal volume by around 1 l; as the external shape is the same between the two versions.
So this undermines the theory that version B was a short-term answer (which might very well turn into a long-term "fix") for tank-spreading issues in Europe. More material means thicker walls, thus more rigidity against deformation.
I think that the bottom part of the tank receives much more additional material, thus reducing inner volume of the lower part of the tank more than the upper part. This might be on purpose ... the lower parts of the tank are obviously in contact with fuel nearly always, as opposed to the upper part of the tank, which is only in contact with fuel when you have just filled up. This also explains, why the "loss" of internal volume between my first tank and my second tank substantially reduces the distance I can now drive once the low-fuel light goes on.