[ProDigit]

Yes, the engine has most HP and most torque between 6000 and 6600 RPM. More than at 9000RPM.
If you where to go 90MPH on stock sprockets, you'd be doing over 10500RPM or past the redline, very unlikely.

On the 15/28t I often surpassed 87MPH.
I'm currently testing out a 15/27t setup, as the 15/26t setup wasn't favorable top speed wise.
If you have a 2013 Rebel or newer, and are light weight, small in posture, and don't have saddlebags or other stuff on your bike, a 15/27t might be right for you.

Rebelgears.com makes them. There are others that do as well. The stock chain works for upto 15/28t, and need one or two links removed for a 15/25t setup, but will be at the end of the adjustment.

I live in South Florida, a few feet above sea level, and switching to a 15/27 or 15/28t over stock will automatically:
1- Lower engine temps due to lower RPM, resulting in
2- Lower chance of pinging
3- Increase engine life
4- Bring the engine to rev at a 'vibration-less' zone, where the engine vibrates very little.
5- The engine will be running under a higher load, with lower revs, so the carburetor will also run a bit richer than stock, as well as the exhaust pipes will offer less back pressure at lower revs, resulting again in richer riding. The engine will run more at the stoic value of 14.7:1 than at the stock Honda setting of 16:1. It all measures out.

If you want to go even faster, try adding a shot of 2 stroke oil in the gasoline. anywhere between 128:1 to 200:1 is a good ratio. 160:1 seems to be the best ratio for top speed. The piston will slide better on the cylinder walls with the miniscule lubrication; resulting in producing yet less heat.

Just make sure you have Honda GN4 oil in your crankcase, as the modifications stress the clutch, and regular car oil may cause clutch slippage at these peak stresses.