Disclaimer - I have not used the WCP swerve module.
Based on the assembly manual, it appears your calculations are correct. It looks like the gear ratio from the motor is 12:1 (72:18 * 24:8). So with a max free speed of around 6000 RPM, you end up with a module rotation speed of 500 RPM.
By running the modules at this high of a rotation speed, you face 2 potential problems - 1) not having enough torque in the steering motor to hold the steering position (reacting against the drive motor reaction torque) and 2) having a stable control loop. Since the Falcon has such a high stall torque, the first potential problem is not going to be an issue. To address the second potential problem of controlability, the control loop will need to run in the motor controller (with its faster loop time) instead of in the RIO.
Speaking from experience, we migrated our steering control loop into our motor controllers this year and increased our module rotation speed to 330 RPM and we were able to have a rock solid control loop with no jitter and reasonable damping. I believe 500 RPM is certainly do-able, especially if you use the motor’s built in encoder rather than trying to close the loop with the absolute encoder. This would get rid of the backlash between the motor and the module (and the associated deadband in the feedback that leads to over correction and jitter). You would still need the absolute encoder to at least initialize the module position.
It would be interesting to know whether you can close the loop on the absolute encoder and have a stable control loop without slowing the motor down a little. My gut is telling me that it would be marginal, but without trying, it is difficult to say. Damping can get you pretty far, but with the deadband affecting the feedback, it is hard to predict whether it would work or not. But you have plenty of torque margin to be able to slow the motor down if you needed to.