muffler shops should have a reducer they can weld onto the 3" pipe to make it into a 2.5" My question is why would you want that?
I'm no Mechanical Engineer, but I do work with a few who are car junkies.
As the exhaust gases cool down, volume contracts (See PV = nRT from chemistry) and gas velocity gets slower. Thus, having 2.5" ID instead of 3" exhaust near the axle-back is not as big of a flow reducer as, say, having 3" to 2.5" close to the turbo. If anything, we're talking 1 or 2 horsepowers here. I don't think it'll be enough to notice at all.
Turbo XS even used to sell a midpipe that would bolt to their monster DP and taper for use with stock or 2.5" axle-backs.