Exhaust Pipe Size coming out of Long Tube Headers

Hey Everyone- I'm putting some long tube headers on my 2005 1500 Sierra w/ the 5.3. I've been doing some research and found that most people suggest going with a 2.5" pipe for true duel exhaust. With that said the stock diameter coming out of the stock manifolds is 2.75". Would I lose much by running 3" pipes from the header to a magnaflow 2 in (3") 2 out (2.5") muffler? Or should I reduce the pipe size to 2.5" right out of the headers and just go with the muffler being all 2.5" ins and outs? Just figured I'd get a little more input before purchasing the muffler. Thanks