Thanks Paul,
so the minimum value of N for the PER_PLL is 9 for a 2.5MHz refclk. With M=484 we end up with the same frequencies of 192 and 960MHz, and a DCO at 1920MHz. The DCO is the same as before, so Sigma-Delta is still at 4. At the end the refclk is still 2.5 times higher than before, leading to less jitter.
If there are no other pitfalls then this is probably what we will do.
Thanks again Paul for your swift and excellent response.
Regards, Kees