The Circle D is a top notch unit, totally the best build quality and materials.
Chris Sehorn, the owner of Circle D, is a buddy of mine.
They were the first ones out with a performance converter for the 6R80 and there are none better.
Go for the 4000 stall, since it locks up in 3rd gear it won't be too much because it spends so little time in 1st and 2nd on the street.
Add and extra HD transmission oil cooler though.