Some aircraft limit the rates. I believe FBW limits their A320 to 2X, and there might even be a limit based on your FPS. The more base FPS you have, the more you can turn up sim rate. I guess this would be for stability reasons to keep control feedback from going open loop and causing a crash.
There was a thread about this somewhere, and one of the devs gave advice on how you could modify this in one in the config files.