I believe the only way to upgrade to the 6 speed on the GMT800 is via a Holley aftermarket ECU and that requires a 58X reluctor wheel on the crankshaft, which means pulling the engine and complete disassemble. This on the LS-based engines, and I'm assuming it still applies to the 8.1 since it's kinda of like a hybrid but still uses LS electronics. Maybe you can call Holley and ask them?
What gears do you have on it? 3.73 or 4.10?
I've toyed with the idea of swapping an 8.1/Allison 5 speed on my project 04 Suburban 2500 6.0/4L80e, but it's just thought at this point.
You can do a supercharger, turbo or something else from Raylar such as a larger bore/stroker/cam/aftermarket heads. Lawrence Tolman has modified a few 8.1s. Check out his channel on YT. That's probably a $8,000-$10,000 upgrade on the NA option.
You can get a lot more power on the turbo (especially if you do the work yourself) for cheaper on a HP/$ ratio, especially with these cheap chicom turbos that are so affordable these days.
