Most people are interested in exhaust for the sound, not the performance. If you're looking to add performance, you should start with a better intake and a tune. You need more air coming in to warrant letting more air out.
You're not going to get more air in without figuring out how to stick forced induction on the engine.
Look, MAP is MAP. If there is no depression in MAP at WOT then there is no gain to be had with less restriction. It's not rocket science; on a NA engine if you have a restriction in airflow you have lower MAP -- period. You can thus quantify
the maximum theoretical absolute gain available from such a change (which is never achievable in practice) from such a measurement, and fortunately modern cars have such a sensor in the intake tract already -- thus the data is trivially recordable.
The facts are that there is no intake restriction worth attempting to "fix" with modifications AND removing parts of the existing system actually detunes the resonance that Mazda designed into it, and thus results in MORE dynamic resistance to flow rather than less!
That's easily backed up with data, again, off the MAP sensor -- simply removing the resonator/water trap on a stock vehicle results in materially higher
jitter in MAP which is evidence of detuning
, and on a rolling 5-60 run both fueling and absolute time figures say you lose. Some of that lose is attributable to higher IATs, which are also observed, but the rest is probably from the detuning.
Don't waste your money on intake mods.