The big restriction is in the stock exhaust manifolds. Air will only flow as well as the point of most resistance. Changing to a louder muffler will make more sound but not much power, if any.

When I put headers on my 1994, I gained in efficiency and performance. When I put on a full catback the following week I didn't have any gain but it did sound a lot better.

 

very true, but there's also a reduce back pressure will also help since it's less restriction. From personal experience, I'm running stock manifold and cats and I can notice the top end difference. Not only I, but my friend also who has rode with me before when I had the Flowmasters on.

 

I am just commenting on actual results that I could quantify through dragstrip passes, and could adjust based on temperature, humidity, altitude and barometric pressure. When measuring 1-2 mph over a 1/4 mile or a tenth, my butt meter simply isn't good enough to quantify the difference. Heck, I can tell if it was a "good" pass or not, but translating that into exact numbers is something I cannot do without a timeslip or a dyno graph.