Anything bigger than a 51 with the stock converter is going to have some serious lag. I have a pte 54 with a 3500 stall and wouldn't want anything less than the 3500. A buddy dyno'd his car with a ta 53 and put down around 400 hp and 525 torque at around 19 lbs of boost, but this was with GN1s. I put down a little over 400 and 545 torque with mildly ported steels and about 22-23 lbs of boost.
btw, don't be too afraid of the higher stall converters. I was afraid the 3500 would be too loose on the street, but to tell you the truth, I can barely tell its there. If its a daily driver, look at some of the high stall 9x11 converters with lockup.