I believe the limit is upper 400s to low 500s. The limit for the T trim is somewhere around 580 I think...but you wont get 600RWHP out of a T trim without some serious portwork and ATLEAST blower cams.
When I mean limit, im talking about what they will make on 2Vs.
A guy from MPH made 601RWHP from a novi 2000 (much better than T trim, and ALOT better than S) he had to push about 17.5PSI, custom MPH blower cams, and full exhaust. Think the heads were stock, and the intake was all stock. 600HP is not easy to come by...then all the supporting mods to handle the power like new fuel pump, new tranny, clutch, 31spline, maybe a better intercooler, new radiator, roll bar system for the track and street safety, and your at risk of bending the driveshaft. Tim said that the adverage 600RWHP buildup costs around $30K after its all said and done, if I remember correctly. Needless to say im going to go with a 2.2 KB and keep it around 520-550...thats plenty imo. and its at the limit of the stock tranny if you dont beat on it.