jimmyz will be testing more with the HD4850 @ 750MHz, so stay tuned on that. Check his thread in the BIOS modification section. I'm curious of the same thing as well.
keep in mind that the HD4870 has much beefier vregs on it that will make it better suited for higher clocks and higher volts. Higher core speeds might love the extra bandwidth once you start pushing the core, but we really can't tell for certain until the software tools are updated.