Real gains will only be had by a real exhaust. You want power? Build yourself a race airbox (preferably out of pre-preg autoclaved carbon fiber for weight), purchase a full exhaust in some exotic metal (titanium, unobtanium, etc), purchase a Bazzaz tuning suite if it's available, and pay for a shit-ton of dyno time and someone who knows what the hell they are doing to tune it. It's a 600cc motor that revs to 16,500. It literally can't spin any higher to make power. The only thing you can do is make it get air into and out of the cylinders as efficiently as possible. Even then, I'm guessing 10-12 horsepower more at the wheel for like $3,500. You're better off spending that money on a 520 chain, gearing down, and purchasing a set of Marchesini forged aluminum wheels. If you don't understand why I just listed those items. GTFO.