little update / help for those getting gamma shift issues roundtripping between FCPx and Resolve, after much fiddling, we found the final solution was to Calibrate our screens to Gamam 2.4 (or more alternatively, choose a Rec 709 profile in displays) And have the project settings in resolve 2.4 also.
Now all our outputs match wether resolve VLC quicktime or mp4 via compressor.
Little update, after much fiddling and research across on the Resolve forums, it's all to do with Gamma tagging. Resolve is outputting the files with a different gamma tag in the quicktime than FCPx. Long and short is that these settings have allowed consistent gamma between the programs