FWIW, a friend of mine has a 2001 Silverado 4WD with a supercharged (Radix) 5.3L. He was putting down around 370 RWHP and 410 RWTQ and has run a best of 13.57 @ 99.5. He went through 5 - 5!!! - 4L60E's and 4L65E's (built up, hi-perf and just the electronic version of the 700R4) before going to a 4L80E. He's now running a 6.0L with the Radix and a big cam and only has a few miles on it, but I rode in it today and it seems much better to me - shifts nicer around town, still nice and crisp at WOT. I know 1st gear in the 4L80 is taller (2.4x vs. 3.2x??) than in the 4L60, but I would think you could gear your way around that. It comes down to choosing what will last versus what will perform best. My buddy chose what will last, as rebuilding every few thousand miles (and sometimes just a few hundred) was not worth it.
Also, my buddy's trans builder was very confident - almost arrogant - that he could build the 4L6xE's to hold up to the power. I think the AWD/4WD launches are incredibly hard on the parts. He finally went to someone else and had the 4L80 done. I don't mean to discredit your builder - I'm sure he knows what he's doing - but sometimes things just don't work out.
Ryan
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