I've had mine up to 55, and it wasn't wide open throttle. But didn't check the RPM's. I'm probably below 7200 because I have 29X9R14 tires. And to be honest, with the short wheel base of the Rzr I don't think I'd want to go much faster.
Your top speed of 55 mph surprises me. I had my Polaris 850 Sportsman up to 65 by the speedometer and this 900 Rzr should do that well. I'll have to check it out next time I have some straight smooth asphalt to ride on.