I think 40T is a good gear for HT bikes... You get a little more top speed, a little less RPMs & engine vibration, & you still have enough power to climb hills!! (I weigh almost 200 lbs)
I haven't measured my top speed with that sprocket yet... I'm sure it's somewhere in the high 30's.
It's common knowledge around here that a proprely tuned HT with a 36T can get you around 40 MPH, but you will lose a lot of your torque!! If you have no hills in your town, it shouldn't be a real problem.