There are multiple hardware aspects to video editing performance: CPU, GPU, I/O. Contrary to popular perception, much video editing (esp on H264) is CPU-bound not GPU or I/O-bound. The same codec compression that limits I/O rate out of the camera also restrains I/O rate when editing. However if it is ever necessary to transcode to ProRes, then file size and I/O rates will increase by a factor of eight. So I/O can be important but it's not all-important as often depicted.
It is similar with the GPU. Many tasks are CPU bound and an infinitely fast GPU wouldn't make much difference. That said this is a snapshot in time and software developers are constantly levering the GPU for more tasks. The Metal API now in OSX and likely to be used in the next FCPX update leans heavily on the GPU.
Re cores vs performance, FCPX is heavily multithreaded and will effectively use all available cores -- at least for some tasks. However the i7 has 8 logical cores with hyperthreading and runs at 4Ghz. The higher core count Xeon in the Mac Pro runs at a slower clock due to heat.
For H264 editing, rendering and exporting, the Intel Quick Sync feature is very important. Unfortunately the Xeon CPUs used in Mac Pros do not have this. Mac Pros do have more available cores and the D700 GPU is pretty fast but you are talking a very expensive machine -- and you have to buy a monitor separately.
The current "new" Mac Pro is nearing the end of its design cycle and it cannot easily be upgraded. My suggestions:
(1) Stay with your 2012 iMac if possible until 4Q this year and assess the new iMac which will probably have a greatly improved GPU. My wife has an 2012 i7 iMac and comparing its 64-bit GeekBench3 numbers to my top-spec 2015 iMac 27 gives:
2012 single-core: 3632
2015 single-core: 4553
2012 multi-core: 14112
2015 multi-core: 17665
(2) If you are not using the proxy media feature of FCPX, investigate and use this. It is easy and produces major performance improvements.
(3) If you cannot stay with your iMac, consider temporarily getting a refurbished pre-2013 Mac Pro. It will be a lot cheaper than a new Mac Pro yet if equipped properly, essentially as fast. http://eshop.macsales.com/shop/Apple-Systems/Used/Mac-Pro
You could then sell it with less risk and investment after the updated iMac and Mac Pro are released and you can evaluate them.
(4) If still uncertain, put some video you are familiar with on a portable drive and find an Apple Store with a Mac Pro and/or 2015 iMac 27 which have FCPX on them. Try editing it in the store and evaluate the performance.