Did you ever get any further with trying to get better performance from your IIfx/Jackhammer setup? Did you get to the bottom of your suspected PSU issues?
Thanks for asking but unfortunate no. However, I bought a new PSU waiting for a re-cap, and I tested the Hammer in a Quadra 950 with SCSI2SD V.6, which delivered better results but was not as good as I had hoped. Also, there were other strange issues regarding the compatibility between the Q950 SCSI Bus, the Jack Hammer Bus, and the SCSI2SD v.6 (2021). For instance, System 7.5 and 7.6 had trouble booting and stopped showing icons, whereas 7.1 and 8.1 had no problem. Therefore, before I rule out the Jack Hammer, I think I need to test it with another Hard Disk.