Well, based on years of experience with larger masts, I haven't found any major durability issues with 100% carbon masts (I've broken a few, but then I've broken a few
75% masts as well.
The main reason I encourage using the 100% mast is the difference in weight.
I tried to race a 10.0 + m2 sail in the FE races I did in Thailand a few years ago,
and I could hardly uphaul the rigs with 75% carbon masts in the larger sail sizes.
In my opinion, for masts 490 cm and larger, 100% carbon is well worth the additional
price to keep the weight down, and if you treat them well, and they don't suffer some premature failure during the warranty period, they will last a very long time. I still have
some 100% Powerex 490/520 masts that have been in pretty heavy duty demo service
for almost 10 years.
As far as a 3 cam Free Race GT (I don't know what GT means....?) like the Gaastra GTX
compared to a Sailworks Retro, they will be very much the same in range, low wind power, and most other parameters.
Hope this helps,