This seems to be the category that people agonize over the most. Do you want a deep growl, or a soft purr?

Hi-Flow Y-Pipe

From the engine's dual exhaust manifold, and back to the central exhaust pipe Ford placed a Y-pipe to go from a dual exhaust to a single. If you are serious about improving the performance of your SHO, you've got to replace that Y-pipe with a high-flow pipe. Vadim called it "the single most effective modification for the SHO". Adds 11 horsepower to a 5-speed and 7 horsepower to an Automatic SHO. You'll see even more stunning improvement if you've replaced your MAF sensor. Remember to replace the oxygen sensor while you're at it.

Unfortunately, the Hi-Flow Y-Pipe is no longer available anywhere. Sorry.