I stand corrected, The 6L80E is a different animal, I "assumed" this was the reprogramed 4L80E that the aftermarket first used the term in the early 90's using the 4L80E and "spliting" 1st and 2nd with the overdrive clutch.
From what information I just picked up, I can see it has potential as a "truck" transmission, but not for a performance vehicle unless your willing to change the rear gearing as well, because a 4.02 first gear with 3.42 out back would be the equivelent of a 200-4R with a 5.00 rear gear in first.
I see the possibilities, but just doesn't make any financial sense. If the 200-4R won't hold up for your power level, a 4L80E would be the smarter choice, and that isn't exactly cheap either, but would simpler in total than a true 6L80E conversion and rear gearing change.
Good luck
Kevin.