Fuel/air ratio's

[Adam]

I have a few somewhat technical questions on the fuel/air mixutures of the boxster. I have heard that increasing the air flow through the intake and exhaust ends of the car lean out the mixture. I've also heard that running the car lean can lead to fried piston rings or pistons or something to that nature(please correct this if this is wrong because I can't remember exactly). I have also read that the ECU on our car self-adjusts and recalibrates the A/F mixture when it senses it is running lean or rich. If say an aftermarket intake and headers/cats/exhaust are added would this make the car run lean or would it adjust for these modifications? Could these mods harm the engine if the computer is not reprogrammed(ie fried rings)? Would the only way to truly tell what if it is running really lean be placing a A/F sensor somewhere downstream of the engine?