with 1280SP all you need for 2TFlops (assuming r6/7xx style shaders of course, if it sticks to rv770 organization that would likely be 16 clusters, each cluster with 16x5 and quad-tmu) is 782Mhz. That should be easily doable, rv790 did more than that clock stock already, and rv740 could be very easily pushed to that. So if both cards should have more than 2TFlops maybe something like 785 Mhz for the Pro (remember 4850 is exactly 1TF and not one flop more) and 950 Mhz for the XT (this is same clock afaik as the highest clocked 4890 shipped with, and if this would be just a shrink should be possible) could work (that difference between the two sounds a bit small though).
The rumoured die size of 300mm² sounds a bit high for this though I guess maybe, like rv790, it would also include a decap ring (or other similar measures which increase die size but don't really do anything functionality-wise, "only" help to achieve higher clock frequencies).