Tey,
The bike is fine. if you need some umph, then Chris is right, but chain and sprockets are cheap, under $200, if you don't want to wait till they wear out (10k minimum unless you sabotage yourself) I'll even help you swap them out.
If you are tired of the 300 already, then screw us and get a 500cc or a 600cc. None of us is your mama. Ride what you want to.
But like your buddy says, you'll may get into a world of hurt, the more HP you have (and torque) on tap, if you make a mistake.
I will say, riding a high cc bike, teaches you throttle control and self control.