The RC51 stock rear ratios were way too high for street use.
It required more clutch slippage from the lights and 1st gear was good for like 80 MPH.
The 15/41 ratio is a real good all-round ratio for street use.
Only a minimal amount of clutch use is needed to roll form a stop and yet you still have plenty of top end.
It allow the engine to be more in the power range.
It drops the indicated speed by 7 MPH, so you'll actually be doing 7 MPH below indicated speed. That can be corrected by a Speedo-Healer if it's a concern.
Others have tried 15/42 as well.
Don't go down below 15T on the front sprocket as it can cause premature wear on the sprocket and chain.