You're confusing little b with big B. (Note that as a general rule parallel busses are usually measured in bytes, not bits.) SCSI-1 is 5 MBYTES per second. Technically most older Macintosh SCSI implementations are limited to a maximum theoretical speed of around 3MB/sec because they use the slower byte-at-a-time asynchronous mode, because of said lack of DMA. In some of the really cruddy implementations like in the Mac Plus, other limitations slow it down to only a few hundred K per second.
Nonetheless that's faster that 500kb/s. Heck, by coincidence 500kps is what the Macintosh *floppy drive interface* operates at. Machines much older than a Powerbook 500, like an SE/30, are able to push more than 1MB/sec with a fast enough drive.
(Of course, it should be easy enough for the OP to benchmark his machine.)
Nitpicky I know but a 6502 bus at 1 Mhz can only transfer about 500K a second. (That would be the CPU sucking NOPs or DMA. PIO would of course be slower.) The bus has a 50% duty cycle.
(EDIT, again: Well, actually, apparently the IIgs *can* run the bus at a full duty cycle. From the Apple docs:
Code:
Features and benefits include:
* DMA Data Transfer -- on the Apple IIGS computer, data can be transferred at
a rate of up to 1MB per second. On the Apple IIe computer, data can be
transferred at a rate of up to .5MB per second. (...)