Daniel,
Load those GT's up with as much weight as you can and shoot them bare shafted and see what you will get. You'll be surprised what they can handle.
As an example, just this weekend I loaded up a CX Terminator Select shaft Fl with 50 grain brass insert, 160 grain field tip on a 75 grain steel adapter and they flew great out of my 60" Griffin 53@28. I'm looking to load them up even more but have to get 100 grain inserts and 125 grain adapters.
Not sure what weight and FoC you are trying to achieve but go heavy first and play with weigh with the bareshafts before you make any cuts.
I posted my findings in another post but here they are. What is surprising is that these heavy EFoC arrows will show extremely week on Stu's calculator, but will fly like laser guided missiles.
Been doing some tuning for my new 60" 53@28 John McCullough Griffin with CX Terminator Selects 45/60's. I draw this bow a little over 28". Initially, I shot a FL bare shaft using the stock insert with 235 points. Shaft showed weak at full length so I started trimming a shaft back to where the BOP was 30.2". Overall, the shaft seemed good so I fletched up two. Later I trimmed one of the shafts for a BOP of 29.92". With both these set-ups I was getting good flight and the bow seemed PDQ and shot where I looked.
Today, I decided to start from the beginning and see what happens with these CX Terminator Selects. Took a full length fletched and bare shaft and loaded up the shaft with 50 gr brass insert and 235 gr points (using 160 grain point and 75 grain steel adapters) for a total head weight of 285 grains.
To my surprise both shafts flew extremely well. Both shafts ended up side by side with the bare shaft 2" to the right. Observing this I took my other two cut shafts and loaded them up with the same point weight and brass insert. All I can say is WOW! These arrows with this bow are like a lazer guided missles.
What truly baffles me is when I plug these shafts into Stu's calculator they all show dynamic spines being extremely weak for this Griffin's Dynamic spine of 59.7#. A spread of 15.5#-28.7#.
Here is a breakdown.
FL Shaft 32.874" BOP
612 grains FOC 22.1% 11.4 GPP Dyna Spine 31#
30.2" BOP
587 grains FOC 23% 10.9 GPP DSpine 42.8#
29.92" BOP
584 grains FOC 23.1% 10.9 GPP DSpine 44.2#
Mike