Mainly by opening the exhaust you are removing back pressure restrictions and allowing the engine to exhale easier. The true benefit is to allow for future mods that require more air intake, greater boost pressures, and more elaborate PCM tuning. The better an engine can breath in and out, the better performance gains in HP as well as MPG you can achieve.

HP gains are marginal, but you can expect 5 to 15 HP. It'* not really a true gain, your actually removing HP loss by removing the back pressure. But, you can still feel the giddy up and go.

There are lost of other engine, tuning and driving factors that influence MPG, but I did see a slight improvement when I gutted the air box and opened the exhaust.

A converters life expectancy is based on fuel/air ratios (exhaust gas) and the heat that the engine exhaust outputs. If you keep you car tuned and replace O2 sensors when they are defective, you should not expect to have to replace a cat for the life of the vehicle.

The benefit of a hi-flow cat will be lost if the rest of the exhaust system is restricted. My entire exhaust rebuild ran me $690! If you can handle the costs, do the entire exhaust. Additionally a set of headers will help as well. Those are next on my list and will be installed in about two weeks.

I dropped about a grand in to my exhaust. Stainless TOG headers were a great investment. 3" down pipe, 3" free flow cat, 3" to the resonator, 2.5" stainless back to a Flowmaster series 80 muffler and two 1.75" tips. That don't include the 1.9:1 high lift rockers. There are a lot of opinions, mine is open the exhaust as much as you can, then mod away.

I'll be in it a grand plus after the SLP headers. This friggin mod bug has turned my car into a stripper. I keep throwing money at her, but I'm never satisfied.

Isn't that the way it always goes. My wife hates me for it.....
You get 'em modded the way you wanted and turn around and score something else and start the process all over again....