OK, so since no one was able to answer this question for me, I'll answer it myself. I upgraded my PC-3000 Express system from running an i5 with 4 cores to a Ryzen Threadripper which has a whopping 16 cores. Now I'm able to image drives over 120Mb/s(probably the drive's own limit) and only hit around 30-40% CPU utilization. So it looks like you need around 6-8 cores per imaging task to be able to handle the imaging at full speed with no slowdowns from compression.
I'm glad I went with the 16 core since I'm usually imaging two or three at a time on this station.