Following the first test, I've made a new drive train.
The autoshift, clutch, and gear ratios have all been changed.
Looking on Porsche forums it seems these cars had about 20mph chopped off the top for increased acceleration. I used the official ratios from a Porsche short gear ratio transmission but used the final gear from the road version for a little bit of a higher top end.
Hopefully all this work fixes the vibration issue.
Visually I need to add new windows so decals can be shown in them and retexture the steering wheel and interior plastics.
Let me know how it feels.
Download tmw porsche 996 carrera cup rar (sharemods.com)