If you're stock I seriously doubt you're running too lean. Our factory ECU will pull enough timing to nearly kill the car if you lean out too much. Post pics of the damage if you have any. Oh, and about getting 250 out of a 420a without some kind of power adder, no. Trust me. But, I say, build it. 4G63's are a dime a dozen; Be different. You'll spend a lot more, have more headaches, but, you'll learn more, and be a much better mechanic because of it.