Sorry about that. The specs of my iMac is
Late 2015 27" 5K
i7 4.0
512SSD
32GB RAM
R9 395X 4Gb
Your R9 395X is quite a bit faster than any MBP GPU (3.7 vs 2 Tflops). As for the CPU, every new MBP hex-core will be faster than the iMac. Also, SSD drives should be almost twice as fast on new MBPs, however that may not be so significant in real world performance, unless you deal with very large files.
So, if your workflow is GPU related, iMac should still be faster. If it's not GPU dependent, any of the new 15" MBPs will be faster than the iMac for multi-threaded tasks (and slower for single-threaded tasks, but in general, MBP will be faster). Older MBPs will be slower.
For FCPX and Logic Pro, any 2018 15" MBP should perform better.
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That's the guess I'm planning on - uncertainty was mainly due to desktop components performing better than notebook it might not be like for like?
Well, the main difference is thermals, so yes, laptop components use less power and throttle faster, etc. However, the 2018 15" MBPs will still be faster for your workflow.