Im picking up a set of mid length headers and the matching y-pipe. The merge is horrible and is a slip fit from the respective sides of the 'y' - meaning I can easily swap in a Flowmaster merge if I want. The car has simple bolt ons and may get a small cam soon, otherwise stock. The Y inlets are 2.5" and will be mating up to a 3" intermediate pipe

Would there be any noticable difference in going from a 3" outlet merge to a 3.5" or even a 4"? (obviously with a reducer to mate with the intermediate pipe)

TIA

 

If the larger outlet goes into a equally larger cut-out or dump/cat-back; then clearly yes!
If you're just reducing back down to 3" then the gains are a lot less, but it'll still sound better and you'll still gain a little power.

 

Dumb question but how would it affect sound?

 

Less rasp to none, read this:
https://ls1tech.com/forums/generation-iii-external-engine/1076819-thanks-patrick-g-nice-gain-flowmaster-merger-y-pipe.html

 

This^^