I also have had it on my 94 for almost a year. Been very happy.
A flowmaster, or any other high performance muffler, or exhaust system helps to increase air flow from your engine. Getting more HP, & performance out of an engine is all about air. The more air you can get into an engine(intake mods, superchargers etc) & the more air you can get out of the engine(exahust systems, headers hi flow cats, etc) the more HP you will get. A good muffler, or exahust system like the Flowmaster will let the engine breath better, & increase performance. 1 of these muflers will out perform a stock muffler any day, because the stock muffler has lots of little baffles inside it to restrict air flow. This is to cut doew on noise, & make the truck quiet. A good high flow muffler will also increase the loudness, & tone of your exahust. You will get a low rumble type of sound like a big V8 makes.
Any muffler shop can install these. Either the muffler only, or a complete cat back system like Flowmaster's force 2. Also a catback is named this beacuse it's a complete exhaust system that replaces the muffler, & all pipeing from the catiliac converter back.