i was wondering about getting some extra hp by "tricking" the air sensor on the Civic EX engine into dumping more gas into the engine and wanted to know how bad my mileage would be afterwards, how easy it is to reverse (as in, how easy it is to reinstall the original sensor), and if it would have serious negative effects on the car's reliability and also if carbon deposits are a problem. my car is an 03 EX with a CAI. any information is appreciated.

There is no reason. You WILL not get added HP.
Go get a header and high flow cat if your looking for "better" performance.

balls.

balls.
LOLOLL!!!!!!

as BrodyP said won't help.