The Sandy Bridge Xeon should be even faster as it will be a 8 core CPU. ( so i guess you can customize it to a 16-core Sandy Bridge Mac Pro )
The processors used in the 2010 Mac Pro have all been replaced by Sandy Bridge E5-1600 and E5-2600 Xeons. These are what Apple will use in the next Mac Pro, probably giving 4, 6, 12 and 16 core options. The Ivy Bridge processors coming out soon - June is the latest date by the way for the E3 Xeons - only offer better performance than one processor option Apple should use and the platform is still weaker overall.
Ivy Bridge Xeons suitable for the Mac Pro won't be available before this time next year.