Yeah, except that most catbacks (and 3" downpipes) neck down to a 2.5" donut gasket anyway. Might as well be 2.5 the whole time and save the turbulence going from 3" to 2.5 then back to 3"
Our downpipe will be ending in a 3" flat flange. Hopefully there are a few choices of LGT catbacks out there w/ 3" flat flanges also. No donut here. No 2.5" section once the piping steps up to 3"....
Jeremy