The best air would be air from outside the car, and not the hot underhood air.... An air cleaner sealed off to the hood and a cowl induction scoop works good, as does ram air. Most everybody runs the open element air filters, but they certainly don't draw in the best air... Cooler air makes better power, so if you can seal off the air cleaner and draw outside air it will make better power.... How much more??? Dunno, but probably not a whole lot. Would be fun to play with it on the chassis dyno some time though...... I'd guess maybe 5 to 7 horsepower difference, maybe more on a hot day when the underhood temps really get up there....