You won't get 10 MB/s with a Fastlane or 4091 and a Cyberstorm MK2, but 4-5 MB/s and some 90% free CPU (according to RSCP).* Raw transfer rate aside, filesystem benchmarks (like SysSpeed for example) are just as fast with the 4091 as they are on my accelerators with onboard SCSI, so the system "feels" very nippy in general use. For kicks I've even transferred a ton of files between my 4091 and DENEB in DMA mode (a no-no) and it "seems" to work OK with no file corruption or lockups.
Bottom line: Cyberstorm MK2 and 4091 work very well together, no complaints here. I'm using a CF AztecMonster on mine, also tested real SCSI HDD's and CD drives, all work fine.
The FastATA obviously has a higher transfer rate, but it eats all your CPU in the process - no thanks.
*The Fastlane can do a bit better (maybe 7 MB/s IIRC) if you fiddle with the settings, but then it eats considerably more CPU. Both of these cards perform better DMAing to motherboard RAM than accelerator RAM (some hardware quirk), but I digress.
edit: It just hit me, you might actually have an easier time finding a SCSI module for your MK2 than a 4091 or Fastlane, or at least equal chance. You probably know that the MK2 SCSI module won't work if your accelerator is overclocked, but is obviously the best performing option of the 3.
I have gotten 8.5MB/s off the fastlane z3. This is with the latest 8.5 roms(most the fastlanes out there have old roms).
There are 2 different scsi modules for the cyberstorm MKII, one particular scsi chip will easily work fine with overclocking to 66mhz. I seem to recall the other is his and miss. One had a Qlogic chip and the other a LSI if memory serves. Benchmarks rarely consider CPU usage and its my experience that scsi on the accelerator almost always wins over anything else for lowest cpu use. Fastlane Z3,4091,etc come in second. As you say in daily use its hard to distinguish.
