Just installed the stock air filter with a K&N Filter. Hopefully I'll get a little more MPG and have even better performance. I have installed them in my prevous BMW's. I like them. I think they really do what they claim. More airflow.
Same here. Slightly better throttle response and if I recall correctly, a little more intake growl. I've used K&Ns on everything from bikes to cars. No notice on the MPG. Probably very slight improvement which is hard to measure since going on the highway route one additional day per week could improve your mileage.