reed valves and spacers??

Reducing crankcase volume has the benefit of increasing the pumping efficiency, so is advantageous all through the rev range. However, the biggest advantage is in the mid range, not the top end.

As the primary compression goes up, so do the pumping losses.

Once the engine has peaked (torque) you're then on a loser, as eventually the high compression in the crank case will outdo the work done by the piston so you can hit a wall.

Look at the difference between a MX engine and a road racer. The MX'er will usually have a smaller volume case as it needs to work at the lower but wider rev range.

It's very in depth, but just increasing the primary ratio isn't always the answer, unless it was too loose to start with.