So, what can we conclude. SCSI is a nice thing, because the multitasking allows you to work with multiple harddrives at the same time. One harddrive is very fast, but not the speed of your SCSI bus. To get the most out of your SCSI bus, you need to combine multiple HD's e.g. with Raid. When you have an environment which is mostly sequential, then setting the blocksize higher than 64KB will give you very good performance. Combining 3 or 4 HD's will multiply the speed.
Random environments will need to take into account that the speed of the controller is not an issue when looking at the speed of your datatransfers. The speed of the harddrives is so low, that any controller can accomodate the transfer..