Well, more CPU speed never hurt, if you ask me. With 4K workflows it's definitely good to have more, along with faster GPUs, the latter of which is underpowered in comparison in an iMac.
If you're just using it for backups, a USB 3.0 drive is plenty fast.
I hate hate hate hate using any OS from Apple. It always does my head in; I'm guessing there will be people who are on the opposite end.
There's always that one Linux guy... :P