Yes, the SCSI controller will be faster than any IDE controller available to Amiga, as well as lower CPU load than the IDE controllers.
Personally I'm using 1 harddrive and 2 CDRW drives with my Amigas, and there aren't any problems. Since you only have listed 4 devices, there isn't a problem, since the SCSI controller can have up to 6 devices connected.
