for your exhaust idea i would double think about the straight pipe, you'll loose all of you non existing low end torque, decrease mileage. You'll have to get on the skinny more to get going. (I hate the sound of any vehicle from over 1/4 mile away pushing less then 300hp)
Have you looked at Magnaflow or even Borla. I would go with a straight through muffler (Turbo Muffler) over a chamber muffler for many reasons. Chamber mufflers use chambers and deflect the sound in many directions. This causes drone in the cab and vibrations that get annoying. Flowmaster are made out of mild steel and will rust out over time. Magnaflow's will never rust as they a 409 Stainless Steel. The Turbo muffler offer a prefer-rated pipe surrounded by a packing that absorbs the sound creating a more even and linear sound. The sound waves are absorbed rather than reflected creating zero drone and sound excellent.