To me it seems like you're thinking of it somewhat backwards. You'd build a motor to make power, not to run RPM. If people could get the same horse at 6500 RPM as they could at 8500 nobody would run 8500 because the stresses and costs are much greater. If a motor is set up to run 8500 with proper cam, compression, valving etc and has proper lowers, then yes, it will make more power than a similar motor which is capped for whatever reason at 6500.
Motors become less efficient as they spin faster. Cylinders tend not to fill as completely and frictional losses multiply. Power is defined as work over time, so even if your motor makes slightly less power per stroke at 8500 RPM (assuming it can run 85), you're running 25% more strokes per time unit. Horsepower drops off when your motor's power per stroke drops off faster than what is gained by running more strokes per unit of time.
Hope that helps, maybe somebody can explain it better-