So, would putting the stock resonator back on cause any restriction??

Would I be better putting something else up there in its place?

Right now, I have 2.5" pipe going to the y-pipe. Would it be better to put a new 2.5" in/out muffler up there instead?

 

Certainly installing anything other than smooth straight pipe causes additional restriction to flow.

What's important is that the catback isn't particularly restrictive on a setup like yours, so the power difference with and without it will be negligible.

IMO when you're talking about mods to the catback, do what sounds the best to you. Plenty of modded exhaust examples on youtube, find something that sounds good to you and get those components.

 

So from the y pipe back it's stock exhaust? That bottle neck from doing that would be more of a restriction then any resonator would ever be.