....I don't know how much workload Final Cut Pro X spreads over your CPU as i don't use it...
I'm a professional video editor and use FCP X every day. Many common FCP tasks are CPU-bound, not GPU or I/O-bound. Anyone with FCP X can see this for themselves by doing those activities while monitoring CPU, GPU and I/O with iStat Menus or similar apps. Frequently all CPU cores are busy.
This assumes the most common video codecs such as H.264, etc. If you're editing non-compressed or raw video then the I/O load will be greater. However most casual users don't do this.
GPU is not unimportant, in fact I'd recommend getting the M295X. It's just that CPU is more important for most common FCP X tasks. Each new version of software leverages the GPU more, so getting the top GPU is a good future investment.
Re i5 vs i7, the retina i7 is 14% faster than the i5. The i7 *also* has hyper-threading which in my tests accelerated rendering to final H.264 output by 30% vs the same CPU with HT disabled. However the HT improvement is highly variable. E.g, it does not help LightRoom import/export at all. By contrast the CPU speed improvements help almost everything.
The CPU in the OP's 2009 iMac does not have Quick Sync. That alone can accelerate single-pass rendering to MPEG-2, H.264 and MPEG-4 by about five times (500%).
Besides the above improvements, each newer Intel CPU has Instructions Per Cycle (IPC) improvements. They are generally modest, say around 5%. However from 2009 to the latest Haswell CPU is several generations -- you might pick up another 15% or so just from that.
Since all retina iMacs have Fusion Drive as a minimum, there will also be HDD speed improvements vs a 2009 machine, no matter what disk configuration he chooses.
Just a plain base retina iMac would be a lot faster than he has now. However I'd suggest the upgraded CPU and GPU. For HDD, even the 1TB Fusion Drive is a lot faster than what he has now. Depending on remaining budget he could consider SSD vs FD and the size/cost tradeoffs of that.