How much speed increase would I get going from a:
Intel Core i7-2600K Sandy Bridge 3.4GHz (3.8GHz Turbo Boost) LGA 1155 95W Quad-Core Desktop Processor BX80623I72600K
Intel Xeon E5-2650 Sandy Bridge-EP 2.0GHz (2.8GHz Turbo Boost) LGA 2011 95W 8-Core Server Processor BX80621E52650
No hard data yet, but my guess is the difference is negligent. Yes, more cores, more L3 cache, but a significantly slower clock speed.
Definitely not worth the price. You get a much better performance improvement from a less costly i7-3960X, especially if overclocked.