You have to view an air filter as a restriction, the cheap ones that are made from paper inner and foam outters will restrict too much air and cause the engine to run rich, at that point you lean it to get the mixture right. But the performance it horrible because it's starving for air. The main problem with the paper type filters is they trap moisture in the paper element and clog it up. The better ones are dual foam type that you oil, for my 1/10th scale I run Mugen Seiki filters that are for 1/8th buggies. More air flow is less of a restriction and the needle setting stays more constant after longer runs through rain, dust and mud. When I have a change in performance, usually it's the filter being clogged or a bad plug than my needle setting.
Motor savers are fine, but get the ones that restrict less and filter the most, the screen will be a problem with clogging, I was pitting for a friend in big-block monster trucks and his truck went rich durring the 45 minute main because the Motor Saver got clogged up on the screen, I ran to the trunk and grabed a new filter assembly from my buggy and poped it on the truck a few laps later and he ended up running just fine the rest of the race. The motor saver looked like a dirt popsicle, you could barely suck air through it.