I hope to have a chance to test a Mac Pro modified to use the 8-core 3.3 GHz CPU sometime soon.
For my uses, an 8-core running at 3.3 GHz (Turbo Boost 4.0 GHz) is better than even a 6-core at 3.7 GHz, because there is one task in my workflow that makes heavy use of all 12 CPU cores in my 2010 Mac Pro.
The extra expense of the 3.3 GHz 8 core CPU is a modest improvement that is clearly not worth it for most uses and users, but since I slog away all year long on my Mac Pro, anything that gives me a bit more grunt is worth it; my 12-core 3.33 GHz machine has served me well for 3+ years.
As shown below, the top green line is the 8-core 3.3 GHz CPU. It offers performance as good as any of the other CPUs up to 4 cores, a tad less than the 3.7 GHz 6-core for 5/6 cores, but it extends its performance out to 7 or 8 cores thus providing about 1/3 more computing power over the 6-core CPU for tasks that can use 8 cores. Plus it has 25MB cache memory, versus 15MB for the 3.7 GHz 6-core vs 12MB for the 3.5 GHz stock 6-core.
The 10-core CPU looks like an interesting alternative to the 12-core, offering notably higher performance up to 10 cores in use.
The good news is that OWC has found that both the 3.7 GHz 6-core and the 3.3 GHz 8-core are viable upgrades, and the 10-core 3.0 GHz also.