I read an artilcle before which ran some tests for aftermarket air filters, it turns out they let in more dirt into the engine than an oem filter (in the test the oem was ac-delco i think), test results indicated that oem filters were still the best and the aftermarket filters let in more dirt, probably also more air, thats why you get a little 1-3 hp gain.

I forgot the link to the article, but I would suggest, stay stock.