WHP will not break the transmission.  Torque is the problem.  A 600 whp, 450 ft/lb car is a lot less likely to break the tranny than a 600 whp, 650 ft/lb car.  We've seen the ECU torque limiter set to around 500 ft/lb.  It's going to take bigger turbos and full ECU tuning with the torque limiters removed before we find out.