The default turbo for stage 4 is the GT3071, but can still use the GT28 (as that's what APR previously did), there is no point me going too far with front wheel drive as the traction will really start to be an issue.
The main thing Stage 4 brings over stage 3 is the full low pressure fuelling upgrade, internals and gas flowed / ported head, in the US they are seeing 480+ wheel HP
With the new 235/40/18 semi slicks I fitted today (Federal 595 RSR's) I no longer have the same 3rd traction issues as before, and maybe with some wider fenders then I can go to 245 or 255, but as you say I have full slicks for the track (which are 240/650/18's).
My current thinking is to do the rods and then recalibrate to see maybe another 15-20bhp, and if required do the LP fuelling stuff if that starts to become the weak point.
At present the logs show the fuelling is running really well on my car (injector runtime and LP pressure still have plenty of head room), timing is OK and the turbo isn't at capacity either.
