The two machines tested here are the fastest possible CPUs as of late 2015.
Canon 50-megapixel raw file converson to TIF
This test is a real-world task using Iridient Developer, which is efficiently multi-threaded.
While the 3.3 GHz 8-core Mac Pro wins here, the iMac takes only 17% longer in spite of having only 4 CPU cores and one GPU, which is a disappointing performance for this very high end Mac Pro (8 CPU cores and dual GPUs!).
Convert 52 Canon 5DS R 50-megapixel raw flies to 16-bit TIF using Iridient Developer late 2015 iMac 5K 4.0 GHz M395X vs 2013 Mac Pro