[Stuff about the connection between IDE and IDA deleated]
If IDE speed come from IDA WHERE does the 8.3MB/s sighted for IDE come from?

SCSI is not complex. It is just the way the industry uses and talks about it.
There are THREE key differences in SCSI; the controller chip, the port, and
the software.  THAT IS IT.
Let us look as SCSi in from THIS stand point.

SCSI-1: asynchronous and synchronous modes {SOFTWARE SCSI DRIVER ONLY}
 asynchronous is slower then synchronous mode {only 0-3MB/s vs. 0-5MB/s}
 synchronous speeds can be reached by most SCSi-1 divices with a
 rewrite of the software driver {As is the case for the Mac Quadra.}

SCSI-2 {8-bit}: THIS is the main source for the confusion. This differs from
 SCSI-1 ONLY in the controler chip in most machines.  In the Mac and some PCs
 this is called 'fast SCSI-1' because it uses SCSI-1 ports and software drivers
 AND can produce SCSI-2 SPEEDS through SCSI-1 INSPITE of this even in the 
 slower asynchronous mode.  Average speed in asynchronous SCSi-1 mode 
 4-6MB/s with 8MB/s{See in both Quadras and higher end PCs} Synchronous
 mode just allows a higher burst rate {10/MB/s}

SCSI-2 {16-bit}: TWO versions-Wide/Fast.  Wide SCSI-2 requires TWO things
 over 8-bit SCSI-2:  a SCSI-2 software driver and a wide SCSI port on the
 machine and the external device.
 Fast SCSI-2 also requires TWO things over 8-bit SCSI-2: SCSI-2 driver
 software and that the RECIEVING devise support 16-bit fast SCSI-2.
 Speed of both is the same: 8-12MB/s with 20MB/s burst.

SCSI-2 {32-bit}:  Also know as Wide AND Fast SCSI.  Over 8-bit SCSI-2 this
 requires: SCSI-2 driver software, wide SCSI-2 port, and that the RECIEVING
 devices ALSO have a 32-bit mode SCSI-2 chip.  As expected this is VERY
 expencive.  Speed: 15-20MB/s with 40MB/s bursts
 