I can only go about 3.7 to 4.5 with my 7' RFM on my tractor and keep a decent cut (has digital speedo). Any faster and I have some grass sort of stand back up later. With the Grasshopper front mount ZTR mowers I have with 61" cuts each, I paced them at 11.1 to 11.3 MPH while still keeping a great cut. I guess that's why I spent the money for a couple of them rather than mow with the tractor. Even with smaller widths, they mow considerably faster; especially when you take the quality of cut into consideration.
I doubt that the RFM's have anywhere near the blade tip speed of the ZTR's based on what I've experienced. I know some companies post the blade tip speeds, but I don't recall any.