With a stock or even lighly modified engine, the OEM head pipes are about as good as your going to get. Honda nailed the length and diameter of the OEM head pipes for the RC51 (there good up to the mid 130 HP range).
With a more modified engine you would need a larger diameter head pipe. I have been running Full Akrapovics for the last few years (good up to the mid to upper 140 HP range). My next exhaust system may have even bigger head pipes, some where in the 60+mm range.
You may still find the Full Akrapovic SP2 system in stock laying around. It will work fine on an SP1 but some grinding of the exhaust outlet flanges will need to be done.
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