flowmaster mufflers dont flow very much better than OEM. a strait through muffler like a bullet one or magnaflow moves air much better. when i deleted my cat on my camaro i felt a TINY difference, but when i did on my truck i felt nothing, probably because the truck is so much heavier and that small amount of power is not noticable. your setup with the 3" I pipe is like the one i have on my camaro, cept i have no cat. its LT headers, 3" collectors into 3" Y into a 3" I pipe, then single magnaflow with dual 2.5" outlets. ditching the cat and picking up a higher flowing muffler is probably the cheapest way you can do it, although nothing beats true duals for performance and sound.
or you can do a cutout right before the cat and have no resistance when you want it