*cough* Actually, aftermarket pipes are not always less restrictive than the stock system (ask me how I know). However, in general, a slip-on will flow better than stock and a full system will flow better than a slip-on. Given the fact than most stock bikes will benefit from a change in jetting, I recommend rejetting any time you make a change to the airflow characteristics of your engine.
Rarely is the question stupid, but sometimes the answer is
you need to run everything you read online through your own personal BS meter to determine if it makes sense to you.