I might have found a piece of info that might alow higher clocks on the core. What we are seeing now are R520 rev A14 and A15. Dave at byeond3d states theres a A23 out there too.
http://www.beyond3d.com/reviews/ati/r520/index.php?p=11
Read whats on this link about A23. The queston is what is A23 and why are they not being used right now? Could this be what a X1800XTPE will use?