[likaroc13]

No idea what size, but if I'm not mistaken the pipe diameter of my stock Y-pipe (manifolds to cat) is smaller than the pipe from the cats to muffler. Mine still has stock cats, so I don't think anything was modified there. My plan is to eventually do a little larger diameter Y-pipe tucked up higher since I'm lowered.

If you have an aftermarket intake, headers would probably benefit. I ran an Edelbrock Performer intake & Gibson stainless shorty headers (& exhaust) on my old '93. With the stock intake on my Tahoe, I'm not sure headers would benefit much. I read an article in one of the truck mags where they individually added bolts-ons, showing how performance was affected with each modification. Some parts are made to work together & could potentially do more harm than good if not used in the right manner. I'm not an engine guru though, but that's just based on what I've read/heard over the years.