[Christ]

If air filters and exhaust have proven gains in efficiency for you, there are other problems most likely with your setup that are causing inefficiencies, or you're not operating the engine as efficiently as you could be.

An engine will only pull enough air and fuel to make the power it needs to do what you're telling it to do. If the engine can rev to 5,000 RPM under load on it's own, it will continue to do so whether or not you put a high-flow filter or exhaust on it. The only difference will be how fast it does it.

Your OEM air filter is not restrictive at all, contrary to popular belief. The K&N filters do flow more air, but does the engine really need it? Do a little math and find out how much air in CFM your engine really needs at the speeds you operate it at. Chances are, the OE filter will accommodate that and more.

The exhaust is a similar bet. Almost universally, when you open up your exhaust system, you lose low-end torque. This has nothing to do with back pressure, which your engine does not need to operate efficiently.

The only way you don't lose torque is if you build the exhaust in such a way that it supports the flow rate at the desired RPM. Remember, your power curve is like a game of Jenga. If you add up top, it had to come from the bottom.