I've read that in many tests the iMac is as fast if not faster then the Mac Pro. Personally, I'd opt at looking to spend about 2.5k on a configuration that includes a 5k 27" display and current chipset over spending over 3,000 on a computer that has not been updated by Apple in 3 and 1/2 years.
As for the MBP in 2016, I doubt it will have a GPU that will be on par with the iMac's.