I would equate using the stock air filter to trying to jog with your hand over your mouth... Same goes for the exhaust system, but I like my trucks to stay all nice and quiet and my days of headers and cherry bombs on my off road racer are long gone besides, all that noise just kills the sound of the Smooth Jazz oozing out of my JBL speakers
My wife had a 04 4 runner with the 4.0 V6 and it got as bad or worse MPG as my full sized Tundra. Here again with the 4 Runner, you are dealing with a heavy brick of a truck and an engine that is working hard to move it... She traded hers in for an Access Cab Tundra (this way I could finally have my truck back) and she loves it. She spends about the same for fuel, less for insurance, and has better power to pass... If I could afford such, I would put her in a M1A1 Tank to keep her safe but yes, I feel better knowing that she in riding in just that much more metal surrounding both her and my step daughter.
Really though it is bottom end torque and not horse power that moves weight. The MPG advantage of the Toyota V8 is its small displacement, but it has variable valve timing and 32 valves to make up for the fewer cubic inches. We also have to remember that with the bigger trucks come bigger brakes (one of your limits on towing weights) and even though the 190 is not huge, bringing it all to a whoa in a panic stop is both quicker and safer with full sized brakes doing all the work.
Now you can install a K&N Filter on pretty much any engine, but I would not do such if you are going to trade it in. The critical thing though is to "track" your fuel burn rates to see if the money you are spending for both fuel and engine improvements is giving you any returns? For even if it is tiny 1/10th of a gallon increase then in time, that change will pay for it's self in better performance. Now I have a spread sheet I can send to you that does all of its own running calculations of you need one.
Dave