In terms of size I think the general consensous has been 2.5" is too big, 2.25" is the max you would want to go, and seeing I won't be doing much else to the engine except for the intake and exhaust, a mandrel bent 2.0" should be fine.

I've also read on other threads that the pipe which goes over the sway bar and X member is restrictive, I would like to replace this also. On the diagram this looks like the section from B to C (can someone confirm). Is it also worth replacing the secondaries (A to B)? I've heard the extractors are pretty good, and don't really need replacing.
Gonna be losing the cat (but keeping an eye on upcoming legislation), so parts C and D will just be straight thru. That section from C to E will basically be replaced with two resonators.
Not too fussed on keeping the twin pipes either. As far as I know the second only opens up at high revs anyways, but a 2" all the way should be able to cope with a N/A 1600 with out the need for another pipe. Any comments on this?
Appreciate any feedback
cheers