Speaking from experience with both welded diffs and turbo cars and autocrossing, here's what I've experienced. Welded diffs seem to work better with cars with slicks. Bias ply tires must have enough give to make it work better than stiffer radials. As for turbo cars, they are far from the best setup for autocrossing. The quick transitions and short bursts make them far from ideal. Just because the SR20DET has the same HP levels as a V8, it's how it makes that power that is key. Torque is king in autocrossing and linear power of a V8 will always trump the turbo "on-off" switch power band in that game! I had a VG30ET in my 510 and it was much better at the hillclimbs than autocrossing, as I was always afraid to "stab the gas" while accelerating & cornering, as boost would hit and the car would come around on me. A welded diff would make it worse, as they make the car push until they loose grip and then you get sideways. Finess, balance of handling & power and smooth driving is the key to going really fast. Of course, sequential turbo cars, like the 3rd gen RX7, aren't as bad, but if you are cornering on the limit right between the transition of the 2 turbos, the same problem can occur with a spike in boost! It's always something! Driving style, vehicle type and course layout always add variables to the whole equation.