I'm planning on going boosted, but it will be a couple years. I want to get a y pipe and new muffler soon and was planning just to run 2.5" from the cat back. Instead of getting this should I just get 3" back? I'm wondering if it will hurt the performance.

 

A couple of years, no just get a 2.5" and if it is still in good condition then sell it.
You will loose a lot of low and mid range torque for very little if any top end.

A 2.75 exhaust is something to consider if you are crazy NA modeed, most likely with the VQ35. 3" is huge for our cars unless boosted.

 

Anything above 2.5" will hurt your low end unless you plan on destroying N/A maxima power levels. It's gonna need LOTS of modding to justify a 2.75".