What Unknown_K said, twice.
Your disk performance is limited by the slowest performing item in the system. The bottleneck.
In this case, the bottleneck has probably been the storage device in every case.
Back when, I had some Seagate ST32550N and ST32550W drives. These were some of the very first Barracuda drives which spun at an amazingly fast 7200 RPM.
The N variant had a narrow SCSI interface (5 or 10 MB/s depending on unenhanced vs. Fast SCSI). The W drives had a Fast & Wide interface (20 MB/s).
They both delivered exactly the same 6MB/s performance when installed on a JackHammer or on the Fast bus of an 8100.
It didn't matter that the interface could do 20 MB/s; the drives couldn't deliver data any faster than 6 MB/s.
If you want a good test of the potential of your IIfx with JackHammer, you need a SCSI drive that can deliver data faster than the interface you're trying to test. More modern SCSI drives probably can do this.
Of course the 68030 may not be able to make good use of the interface. I'm not sure about that.
BTW, IBM, when they sold hard drives, before they sold their drive business to Hitachi, used to include "Media Data Rate" (I think that's right, the first word was "Media" in any case) in their datasheet for their hard drives. This was the rate at which you could expect the platters and heads to actually deliver data, regardless of what the interface potential was.
As far as I know, IBM was the only company that included this bit of honesty in their hard drive specifications, and pretty much everyone overlooked it.
I now have several good boards and several lcs. I managed to completely rebuild two flawless PB 100. Now I have some recapped logic boards and 4 lcd: 3 blacks and 1 good (but with a colums of dead pixels due to a crack in a corner). The 3 black lcds stay black with any logic and the good one is fine with any logic, so it must be something in the lcd circuitry. In my testing i am using the same inverter stuff, I swap only the lcd panel.
Awesome find! Glad it made its way to a collector rather than a landfill.
The book "Revolution In The Valley" is all about the making of the original Macintosh, written by much of the team, most of which has been posted to folklore.org, so here's a relevent story about them switching from the Twiggy to the Sony drives
Whole site and book is worth checking out