David,
Yeah, the network is the part that *****. I'm pulling a solid 450MBps write and read on the Pegasus R4 (4TB in Raid 0) and 2 of my iMacs can copy files from the 1 iMac with Pegasus R4 connect at up to 120MBps speeds across a wired network using a Airport Extreme. Bottom line, 120MBps across a wired network is still faster than USB or Firewire but nowhere near the 450MBps that the 1 iMac enjoys. Since you will have the Pegasus R4 or R6 plugged into Mac Mini servers you won't see any speeds higher than 120MBps. I'm able to enjoy the 450MBps write and read on 1 iMac because my R4 is plugged directly into 1 iMac and 2 other iMacs feed information from 1 iMac with the R4. I do not use the R4 for backup though...my main data is on the R4 and all 3 iMac's edit from the R4 unit. With 3 people editing images on the R4 my speeds do drop down some as low as 60MBps but I would say it's a good 90MBps most of the time which is still faster than firewire 800.
I do actually have Lion on my R4 unit so I am not using my internal HD. I have 3 Backups to the R4 unit, 1 via Time Machine(FW800 HD) which backs up on the hour and 2 via Carbon Copy Cloner(USB drives) that are in the morning(1am and 2am). I also have 2 other Backups off site, one has original files and the other is swapped out every month. I do have other backups but you get the idea...backup, backup and more backups are good.
Oh, I only have my R4 units on a desk and so i'm not sure about rack mounting them. Might be able to get away with Stacking them although they are very nice looking imo and sitting on a shelf is not an eye sore at all imo. I do use 2 R4 units. One as the Main for everyone and one that is on my iMac, I use FCPX and wanted to enjoy the 450MBps speeds so my R4 is only for FCPX only although Lion is installed and I don't use the internal HD. I think the Mini is good but with limited network speeds, I think it's kinda pointless. Might as well let 1 Mac enjoy the fast speeds of the R4 and the other 4 in your case have the 100MBps speeds. For someone who isn't interested in getting a new Mac, the mac mini is a cheap solution so you just got to work out what's best for you.
I haven't had any problems with my 2 R4 Units.