I don't have any experience with that specific controller. It looks great though!

1.5M/s was probably plenty when this thing was a state of the art. However, it lower I would expect. My Adaptec controllers max at about 3 MB/s though. And that's on ISA.

In my recent testing with modern solid state cards I haven't found a 50-pin SCSI controller that was in absolute numbers faster than average VLB-EIDE controllers. The strength of SCSI was always flexibility more than speed.

One think I would check is BIOS shadowing. Advanced controllers are often sensitive for this and if you keep using routines from ROM over ISA then you'll get much slower speed than if you copy their BIOS to RAM. Adaptec cards try to sort out shadowing themselves, but for your card you might need to do it for your card in the BIOS or tweak EMM386.EXE (if you use one).