I've not dug into the contour engine bay too far for turbo kit investigation but you do have a decent amount of freedom for external wastegate placement. It doesn't have to be right before the turbo... it can be sllit nicely at a Y a good deal earlier which might help arranging it to fit. I think the boost control setup for externals is a lot better than most internal gates. I know it's better on my WRX.

Exhaust size is a tricky business. Downpipe sizing is not really a huge deal... where it comes out of the turbo you usually want it to flare outward allowing the gasses to expand. Bigger isn't always better... but in turbo applications it usually doesn't hurt either.

The Up-pipe area is much more critical for sizing. Most people who do custom crap seem to make the up-pipe tubing way too big thinking that bigger is better. The tubing after the 3-1 is what like 2" on the contour headers and I think warmonger mentioned 2.5" which is a pretty good sizing apparenly in practice... and according to what most literature tells us. Make sure you have a smooth transition from up-pipe to turbo turbine too.

04 Subaru WRX "Eurosport bling bling fast and furious tokyo drift" "They have diarrhea of the mouth, and constipation of thought"