I have an exhaust leak and I'm pretty sure that it's comming from my Y-PIPE. I'm a completely stock setup.
Sometime in the future I'll be getting headers (shorties).
My question is, if I stayed with stock manifolds and wanted my Y-Pipe to terminate into a 3" pipe and go through a 3" High-Flow Cat but stock pipes after the cat (basically just the Y and the Cat will be changed at this time and later, Headers, Muffler and way in the future, cat-back piping) can I just use sizers (or whatever they're called) to go from my stock manifold to a bigger diameter pipe down to the cat-back piping, or will I lose a lot of power or create a lot of back-pressure or something? All I'm trying to avoid is buying a Y-Pipe now to fit manifolds and change it later when I go for a 3" header-back system.

 

SLP makes a direct fit y-pipe replacement but it is costy. You are better off trying to patch and or fix the leak while you save for headers. You can go the route you mentioned but all the reductions and such you create will make for a lot of work.