I do not have any experience with the shafts you are asking about, but it sounds to me like typical GT quality control (or lack thereof)! I used to shoot GT XT Hunter black shafts. I quit shooting them because they were all over the place spine-wise. In any given dz .400 spins shafts there would be some as stiff as .310 or so, and as light as .390 or so, with the majority being in the .350-.370 range. Out of 10's of dz's of shafts I put thru a spine tester, I never had one that actually spined at .400!
Bisch