I would fix your first issue! Why are you loosing power? Have you checked your cam timing and run a compression test. They are very easy to do, rent them from any auto part stores and follow the directions. An easy way to tell if you cat is clogged it pick up an infrared temp gun. Measure the inlet temp and then the outlet temp. If they are equal the cat isn't doing it's job. If the outlet is higher your fine.
As for your exhaust idea i would double think about the straight pipe idea, 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)
Flowmaster mufflers are designed for 8 cylinder engines and will drone and sound terrible IMHO on your application. 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.
Edit: and damn you're in Cranston, If you need anything looked at PM me I'm a Mazda Tech at Flood
