from what ive heard and seen, going from the .63 to a .82 or .85 will give you 1 -1.5 MPH in the quarter, im not sure how much gains on ET.
I dont know how much you would loose going back to the .63, i assume you are looking for the smaller housing because the car spools slow.
if it was my car, and i had a issue with it spooling slowly, i would not revert back to the .63 because of the gains the .85 has MPH wise.
if you have a issue spooling the car, i would suggest purchasing the bleeder valve and see how well it makes it spool.
a friend of mine has a fast system that has the ability to record data from the engine, his datalogger shows that his car will go from 9psi launch to 26psi of boost in 7/10ths of a second!!!!!!!
all that with a large 70mm turbo and tiny 3200 multidisc lockup stall convertor that everyone tells him the convertor would be too small to spool this turbo..........
just trying to help out,
BW