Depends on the muffler, most aftermarket mufflers are straight through with pockets similar to glass-packs, they don't restrict flow any, they simply interrupt the sound waves to make it a little quieter.
Now the factory muffler, thats a muffler, baffles, packing and all kinds of other stuff, that restricts flow.
I doubt if you'll actually feel any SOP (seat of pants) difference b/t with a muffler and without, it will probably seem faster because you'll lay into it a little harder so you can hear it. May lower EGTs a bit, but not significantly.
-R. J.