The Messico and stock GasGas pipes can't be beat on a 300 IMO. Other pipes can give you a stronger bottom end but always at the expense of the top end. Thinner cylinder base gasket and removing the powervalve shims may give you all you want but if you still want a little more the RB head mod is less than 100 bucks and worth every penny - better performance, better fuel mileage, and easier jetting.
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