I believe the Soft ROM is just a software extension the speed up the card especially when the card is modified with the 14 MHz jumper.
No, that's something separate. All hardware needs a corresponding software driver. Most disk controllers have this in ROM onboard. Other devices use disk-based drivers (in SYS:Expansion, Devs: or Libs: for instance). With the onboard ROM disabled, the 2091 needs a disk-based driver.
I think, though, that there is an optimized version of the soft ROM for boards that have the 14MHz mod. There may be a related software patch as well for cases where the onboard ROM isn't disabled. Maybe you're thinking of one of those.