How small can the time-steps be with FSI turned on and a high-perfomance PC?

It depends a lot on what other processes you have running on that machine, but also depends a lot on the model complexity and hydro mesh sizes. 10 ms has been done in the past without problems.

Here is our advice to find out how small a time-step can be taken:
Make Gomboc process high or realtime priority in the task manager, kill as many other processes as possible and progressively lower the timestep duration, until you start to run out of realtime (easily to see with the Clock block and the overlay that turns red).
Please note: sometimes the model runs smoothly in realtime and then at touchdown (hull hydro kicking in) or additional foil in the water during tack and gybe can cause the model to slow down
So the baseline should be worst case, all foils in the water, hulls in the water and then try.

See info about Clockblock in BLocks/BLock Reference/Clock.
And information about the overlay led that turns red in Graphics/Overlays. Another solution to visualize if the simulation is runing realtime is to turn the graphics window red using a DataTable overlay (see screenshot).