I gave up on the 7.1.1 + SCSI Manager 4.3.1 + Sync-only U320 drive test. I'd overwritten my boot floppy with an 8.1 boot floppy and didn't want to re-overwrite it. I put a different 10K drive in the Mystic just to get this project closer to completion.

I can confirm that the SCSI Manager 4.3.1 extension does work with System 7.1.1 on the Mystic! Running Photoshop 3 under 7.1.1 without SCSI Manager 4.3, the efficiency indicator shows sync IO. After installing the SCSI Manager 4.3.1 INIT and rebooting, the little dagger appears next to the scratch efficiency showing that has begun using asynchronous IO calls.

Another interesting result is losing large-block throughput by installing SCSI Manager 4.3.1. I was getting about 6MB/s in the ATTO Performance utility without SCSI Manager 4.3.1. After installing the INIT, the results are hard-capped at 5MB/s. I suspect that this speed decrease is as a result of running the SCSI bus at the 5MB/s synchronous mode when the SCSI Manager 4.3.1 INIT is active.

The Finder is much more responsive during file copies, though -- so overall performance should be higher even though bulk throughput is lower.