I once excitedly ran many tests of that famous init, hoping for a big boost in performance. Sadly, I could never measure any difference at all. It's probable that the drivers installed with system 6 (and possibly some earlier ones) effectively do what that init does.
Mike -- Equill has provided a valuable pointer, as usual. A not uncommon cause of slow SCSI performance on the Plus (beyond its inherent sub-optimal implementation) is to connect a drive that has been formatted at a 2:1 or 1:1 interleave. The Plus can't keep up, and access times can stretch out by a factor of 5 or more, giving you floppy-like performance. If that drive was formatted on another Mac, it is almost certainly the case that the interleave was set to a value that will kill performance on the Plus. For fastest performance, keep the drive defragmented, max out the Plus' ram, disable all unnecessary apps, extensions, and run system 6 if feasible. A Plus can't run system 7 with any grace, but system 6 flies (relatively speaking).
Mike -- Equill has provided a valuable pointer, as usual. A not uncommon cause of slow SCSI performance on the Plus (beyond its inherent sub-optimal implementation) is to connect a drive that has been formatted at a 2:1 or 1:1 interleave. The Plus can't keep up, and access times can stretch out by a factor of 5 or more, giving you floppy-like performance. If that drive was formatted on another Mac, it is almost certainly the case that the interleave was set to a value that will kill performance on the Plus. For fastest performance, keep the drive defragmented, max out the Plus' ram, disable all unnecessary apps, extensions, and run system 6 if feasible. A Plus can't run system 7 with any grace, but system 6 flies (relatively speaking).