I Had to use a program called "MyUnitControl" From Aminet to set my SCSI Devices to run in Synchronous mode with a Blizzard 2060 - The speed doubled with that set though I have to disable it if I Plug an external CD Drive in. It has instructions on how to put it into your startup-sequence.
Phase5 Had a utility called unitcontrol on their setup disks, but I wasn't able to find any information about getting it to run or set at boot time, I could only use it by running it manually after boot and it would only change the settings for one use.