having had a pci expansion unit to run DSP's, firewire, USB and other interfaces to keep SCSI to the G3, running a full 24 track count on the coe and 2 farm cards it hadn't much trouble at all.
magma chassis use unique boards and now it's different cables so you wouldn't confuse them with scsi leads. PCI Bus works on total load of all streamed channels, so at 133mhz rate, you can perform quite a bit. what bottlenecks most of all in certain situations is buffering data to storage because of the amount of data bounce via SCSI as an example, it can have a knock on effect. These are good systems if you configure the storage controller inside the expansion chassis and mount drives inside (7 or 13 slot types) where you can install SCSI drives, the raid controller, the TDM boards, etc, then hook this up to your host mac and load a firewire / usb card and that's it. keep the main internal card structure reasonably clear other than vitals.