Originally Posted by DrBacon

Proper belts and belt tension. Unlike a turbo a supercharger is literally mechanically connected to the engine, it will always try spinning at a certain rpm relative to engine rpm, thus moving a constant amount of CFM. The belts are the weakest point in the chain and usually the limiting factor.

The K&N filter is restrictive, but not in the sense that people realize. That filter when clean is capable of flowing enough air to support 15+ish psi of boost but it takes more work for the engine to suck the air through it creating a greater parasitic loss on the engine. When there's more drag it usually ends up in belt slip and thus lower boost, if you can properly tension and use the right belts that slip is reduced/eliminated. This is why you see people gain boost going from a filter to a turboguard, it's not because the filter isn't capable of flowing the air but rather because the drag is reduced so the belts are no longer slipping.

You can have a filtered car and a turboguard car with the exact same setup make the same boost, but guess what? The turboguard car will always make more power since it isn't working as hard to pump the air. Boost numbers don't mean everything.