I have an idea:
Shove some E3's in the lower end Mac Pro
Only have 20 PCI-e lanes. [ most board "fake" multiple 16x slots by sharing bandwidth or chopping down to 8x. ]
E3 has enough for a video card (16x) and Thunderbolt (4x) and that's about it.
Some E7 10-cores in the higher end Mac Pros
The E7's are a generation behind in microarchitecture (although the ring bus cache is relatively modern). The E7's from April 2011 just got Westmere. The Sandy Bridge E7's probably aren't due till 2H 2012 at this point if they have not slid into 2013.
E7's are more expensive than E5's at anywhere near the similar clock speeds. There is no good upside in putting them in a Mac Pro.