There'* nothing about the stock muffler that blocks air flow. But I think we're getting off track. The issue is this: If you put in a turbine to suck exhaust fumes straight out of your engine with the capability to pull out thousands of cubic feet per minute, you are still not going to gain 30 hp. Let'* suppose you actually get the best flowing exhaust setup ever made, and let'* suppose it really raises your horsepower. You're still not going to see a 30 hp boost. We kind of got on the wrong track talking about exhaust. Whether that increases horsepower or not, ultimately it'* the 30 hp figure you wrote above that spurred this.
You need to get headers, a 3" H-body downpipe, a 3" high flow catalytic converter, and then you can leave it at 3" or taper it down to the stock 2.5 to go to the muffler. By the time the exhaust gets to the muffler, it has cooled off and has shrunk a bit, so keeping the pipe at 3" impedes air flow. The only two Bonnevilles running 3" exhaust or no exhaust at all that really should have that are the top two quickest 1/4 mile Bonnevilles, and they have about 500 hp. Otherwise, Bonnevilles have a 3.8L V6 that couldn't possibly benefit from a high flow muffler all the way at the back of the car. You need better flowing stuff right at the engine first.
94 STEi and 99 SLO