its not the CPU that you'd have problems with (there is have a 1366 backplate available), but the surrounding circuitry / heatpipes. The thermalright sits pretty high, so that shouldn't be an issue.
The i7 CPU is a fair bit larger than a 775 CPU, but if the backplate holds it well centered, the base should cover ~ 85% of the IHS. Should be enough to do the job....
OTOH, they do make a heatsink just for that socket... Is there really a design change? Or is thermalright just another greedy corp.?
So all that said, I really don't know. Maybe send them an email?