Some results that I heard from customers when I was still at PTE, were anywhere from 400-500 rpms difference on the spool up times. Now, this could be a result of a bad tune (aka too rich during spool up) or a result of too tight a converter. I have run a 70 P-trim in the past, and I did have a .63 A/R housing. I was running a 3800 stall at zero psi and it spooled it up fast. When I had it upgraded to a 74Q (old school) back in 2001, it spooled up ok. IMO, the .63 A/R housing on a 70 P-trim likes a converter that will stall at 3400 at zero psi and the .85 likes a converter that will stall at 3600-3800 at zero psi on a 231-235 CI built motor.
That 400-500 rpms difference was pretty common on all the turbos that I sold, when going from .63 to .85 A/R ratios. I would normally spec a .63 A/R on all P-trim and GT-Q turbine wheel equipped turbos, if the car was going to be mainly a street driven car with limited track use, and a .85 always for a track only, all out car.
Hope this helps.
Patrick