I guess it depends on the particular model you get. My MSI RX9800 Pro doesn't want to overclock over around 400MHz for the core and 360MHz for memory
And it's not that it gets extremly hot and needs extra cooling - there are some artificats and errors from the very beginning when I try to go slighlty beyond above mentioned points.
I am not saying that it's a bad card but some models are much worse for OC purposes than the other and sometimes it's even impossible to soft mod the card to an XT model