You'll never get 96 MB/sec except to/from the cache on a 36 gig drive. Size does matter - the larger a drive, the higher the density and the more data passes under the heads per rotation. The only time 15k RPM really matters is with rotational latency, since with little I/Os you wait less for the data you want to come under the heads because of the faster rotation. A 2 TB 2.5" 5400 RPM drive would be many, many times (easily an order of magnitude) faster than a 36 gig 15k RPM drive, for instance.

The busses on a PowerMac aren't very fast on their own, so it's not surprising you're not getting good performance. Also, there are two motherboard SCSI connectors inside the case, so even if the drive is internal, the cable could still be plugged in to the bus that's part of the external bus. Double check. The screenshot you posted shows it as bus 0, but I don't remember if internal is 0 and external is 1 or vice-versa.