It's been a long time, but if I recall correctly, you cannot mix pre-Ultra2 devices with Ultra2 and up -- this includes Fast, as they use a different electrical signalling on the bus, whereas Ultra2 and Ultra160 devices can play nicely with each other. You can freely mix narrow and wide devices as long as you segregate that end of the bus, terminate the high lines there, and chain all the narrow devices from there on. You still are limited to the fastest compatible signalling rate on the whole bus though.
i.e. Ultra2 and Ultra160 use LVD which you can't mix with SE Fast SCSI and regular Ultra. Some chipsets will let you mix Fast and regular Ultra, but I'm not sure which. I also think you have to segregate old fashioned SCSI-2 to its own bus (and to be honest it's so slow you might as well stick it on an old A2091 or something)
I also seem to remember that there was a tool on the CSPPC SCSI Tools disk that let you see what the actual negotiated speeds were (or they were even viewable in the ESC-key early startup menu on newer CSPPC firmware revisions).
In real life I never got better performance than 20Mbytes/sec or so anyway even off a nice 15Krpm Ultra320 drive running in Ultra160 mode. The 68060 became the limiting factor in how fast the OS could service the DMA buffers. Running AmigaOS 4.1 might give more of a benefit.