Upgrading the CPU in the Late 2013 Mac Pro to a 10 core from a 6 core. This isn't the fastest mac pro in the world as the 12 core is slightly faster but much better than the 4 core, 6 core & 8 core. Full geekbench & benchmarks
MrThaiBox123 posted links to the parts and equipment required for the upgrade. He used a Xeon E5 2690v2 10-Core Processor which costs about $2200. The quad-core Mac Pro costs $3000 from Apple; whereas, the 8-core costs $5000 and the 12-core costs $6500.
For those looking to save money, purchasing a quad-core now and upgrading to a faster CPU in the future is a very real possibility.
OWC has great video on changing the processor itself which can also be seen below.