Actually too much fuel is RICH, not lean.

Aftermarket filters, if perfectly clean and oiled perfectly, will filter as well as a cheap paper filter. Problem is that they never stay in said perfect condition, leading to potential problems. For the 1-2hp improvement it is not worth it to risk your engine. The only aftermarket filter that I would reccomend is AEM. They use a dry, plastic filter rather then the oiled, cotten/screen filters such as K&N, etc. AEM are supposed to filter to 1 micon. But I am sure that if you can find one that they will cost more then they are worth.