My brother has also been considering upgrading gearboxes.
Has been using 2nd hand Tremec 3550(non TKO).
Would love to move to Tremec TKO 600 (ratio's of 500 are not suitable).
Advantages of Tremec TKO 600 over 3550:
Better ratio's for track, especially .82 5th, useable fifth gear(he needs it). 3550 has .68 5th.
Nice quick shifter so that he can get gears with ease, especially 5th.
Increased torque rating, you've got to think your on borrowed time when your producing close to twice the torque your gearbox is rated for.
And you regulary circuit and strip race.
Upgrading the gearbox will result in faster track times.
Gearbox upgrades don't come cheap and the upgrade cost is going to be a killer and major stumbling block at the moment.