It may be a downgrade from those listed above in terms of CPU performance and a little GPU performance in general. But when it comes to using iMovie, import times will be super fast on the new rMB as its PCIe flash is twice as fast as the PCIe based flash last year. So this machine will import and export your videos extremely fast. When it comes to the actual transcoding when exporting a video though, that is where the Pro would full ahead. By how much, I don't know (have to wait until benchmarks).
If you are editing in iMovie over FCPX though, I would of thought this machine would be more than fast enough for YouTube videos, but I think you money is better spent on the 13 inch 256GB MacBook Pro. It still has the super fast PCIe based SSDs, force touch trackpad, 8GB of RAM... But for the 100 pound more over the rMB, you're getting a larger screen, better battery life, more ports, a much better CPU and a better GPU.