Variants:
Mid 2012: If you're already running a rMBP 15" (Mid-2012) model, it has the NVIDIA GT650M 1GB graphics.
- Early 2013: Not to be confused (iteration of 2012) has the GT650M 1GB.
Late 2013: This model of the rMBP 15" comes with Iris Pro graphics.
- Mid 2014 (current generation, iteration of Late 2013) has Iris Pro graphics as standard.
I've owned the Mid 2012 and currently have the Late 2013 base model Iris Pro unit.
Graphics: The Iris Pro provides nearly the same performance as the NVIDIA GT650M.
- The Iris Pro is substantially faster with 2D perf. in some ProApps (ex. QuickTime exports or media encoding).
- You will not have NVIDIA CUDA which may be useful for DaVinci Resolve (if you can get it to run), Raytracing plug-ins and Adobe suite but, most applications are releasing updates to better support OpenCL.
- Bottom line is if you do not plan to rely on CUDA, the Iris Pro provides nearly the same performance as a dedicated NVIDIA chip with much longer battery life and in general usage does not have the same "UI lag" in Yosemite as the Intel HD4000 (used when the NVIDIA chip is not activated to save power on the Mid2012/Early2013 models).
Other Things to Consider:
SSD: The Late 2013/Mid 2014 models have a substantially faster SSD (~710R/W) based on PCI-E whereas, the Mid 2012/Early2013 units have SATA drives (~420R/W).
Battery life: In my experience ranges from 5 hours to 7 hours on the Mid2012/Early2013 models (due to graphics switching and overall inefficiency) whereas, the Late 2013 model can provide 7 to 10 hours (longer battery usage rating to start with, Iris Pro is highly efficient when not in use and delivers performance when needed).
- The Iris Pro delivers similar performance without compromising on battery life when it's not being used as well as removing the frustration of GPU switching due to iGPU/dGPU on-demand.
Your Scenario:
As you did not specify which 2012 model you are using, I assume it is the 15" rMBP.
- In such a case, you will not see much of a difference at all using either Late 2013 or Mid 2014 with Iris Pro as standard. It is possible, on the GPU side to see a decrease in performance over the 650M that you already have.
- FCPX relies on background rendering and should work "fairly well" on a GT 650M.
- The GT650M or Iris Pro will be able to handle 4K however, I would not expect either of these to be "future proof" or handle the workload anywhere near as well as they can with 1080p footage.
Overall: If you already have the rMBP 15" then it includes the 650M however, if you have the 13" (HD4000) then considering which rMBP to get is up to you after evaluating the aforementioned factors. Personally, I would go with the Early 2013 Model with NVIDIA GT650M graphics as depending on the specific task, it may not be up to par with the Iris Pro (Intel QuickSync transcoding) but, certainly has an edge in many others including 3D or CUDA related tasks (whether as a plug in or in other pro apps).
However, there are certain cases where the Iris Pro even in FCP X doesn't cut it.
See here (pure perf test, 750M vs Iris Pro) ->
http://blog.alex4d.com/2013/10/30/brucex-a-new-fcpx-benchmark/