This TechPowerUp article has some presentation slides with further details about SandyBridge Xeons and first performance claims ("up to 80% increase" compared to Westmere 3.46GHz, with Linpack value going as high as 2,2x compared to the Westmere baseline).
There are also informations about a new technology named Data Direct I/O Technology (DDIO) which bypasses main memory by sending data directly into the CPU cache (whatever that means in detail - slide is pure marketing so far). The presentation claims DDIO to increase I/O performance by up to 2.3 times that of Westmere, reducing latency and allowing system memory to remain in low power state. Probably needed with the highest E5-2600 SB-EP topping out at a whopping 150W TDP...
Intel is definitely serving kool aid.

I hate it when they use drastically inflated numbers. The gain doesn't look that amazing when comparing a cpu with its logical successor. This takes price and tdp under consideration. I'm not sure if the current design of the mac pro is really suitable for 150W cpus considering the desire to keep it silent. Other OEMs often pass on cpus that run this hot when it comes to workstations. I wonder how hot they run overclocked