I have an AHA-1540 somewhere, could I max out the ISA bus with one of those hard drives that has the SCA connector?
The drive is buried somewhere so I can't get the model number, nor can I find the card so far.

To sliderider:

You are wrong when you say that the CPU has to talk to the non-SCSI harddisk all the time. IDE and even MFM have their own controllers, as does SCSI. To get it to do something a command is sent (like read sector xyz), and after a while an interrupt is triggered signalling that the data is available. No CPU attention required during this period on either interface.
That's the hardware side. Under DOS this timespan is wasted in any case, see below.

The difference between SCSI (with controller, not only interface converter) and early IDE/MFM here is that SCSI can write the data directly to memory through DMA or busmaster while in the other case the CPU has to read it from I/O ports and write it to memory. You will still have the access time of the harddisk.

Why the time is wasted under DOS: The INT13 interface is implemented that way. You call it, and it does not return until the data was transferred to/from memory. The time from sending the command until the data is ready is wasted. Even SCSI cannot work around that.

An operating system that loads drivers for direct harddisk access can make use of this time as it is not depending on INT13. On both SCSI and MFM/IDE.

The advantage of SCSI on DOS is really only the faster transfer through DMA/busmaster.