You're in a slightly different scenario in that you already have a T-56.
In that case, I think the G-Force upgrade is the best.
I started from ground zero and found that most places feel that a properly prepped TKO600 may be able to handle a higher load than a T-56 in gears 1-4.
In both transmissions, running in 5th or 6th gear is not recommeded under high torque loads.
While researching the tranny issue, I was pointed to Promotion Powertrain in Ft. Lauderdale Florida. You can speak with the owner, Walt. He thinks it will withstand 1200-1400 hp, maybe more. They get a brand new TKO600, take it apart, cryogenically treat, polish and replace the gears, beef up a bunch of the internals, tighten up the tolerances and add an external girdle that further strengthens the transmission case against flexation.
That's about it. Hopefully it will hold up under high torque/hp loads.
-Bruce