I think I'm good now. I guess I just needed to read more... I had subtitles enabled on my tests. Transcoding with subtitles is a killer apparently.
So, I can now get 4 x 4K movies HW transcoding to 8mbps and 2mpbs streams. I think that's pretty good.
After doing a lot more reading I think I'll start keeping a 1080p version of all of my movies alongside the 4K copies anyway.
I have pretty much the same result as you on the jellyfish tests. 45Mbps works, 50 fails. Though I notice dropped frames/stuttering when watching the jellyfish 35Mbps and higher. Not all the way through, but at certain parts.