You will find that the SCSI bus will run at the same speed as the slowest scsi controller on the chain.
(assuming you meant 'scsi device' rather than 'controller)
I keep reading this from time to time - it's simply not true. You can easily hook up a very slow 2x CDROM and get 20 MB/s from a fast harddrive (wide SCSI). Slow devices won't slow down the bus at all, EXCEPT
- hooking up SE devices to an LVD bus (no LVD on Amiga however)
- while they're in use; transmitting data in a slow mode (async) occupies the bus for a fairly long time, so leaves potentially less time for using a faster mode
