I try and do as much as possible in PP and use Speedgrade only sparingly.
I am not currently editing 4k but may in the future as technology catches up.
I don't have too much footage ATM and try to clean up unused files as often as possible. I definitely want to keep and use HDDs for media/project files and backup purposes, while only using SSDs for scratch/cache. Are you saying that I will see more of a boost in performance if I use an SSD for both project AND scratch/cache files?