What speed is the Mini's Ethernet port?
Subject says it all. Is it just 10/100 or is it gig-capable (not that it should really need it)? The Roamio has a 1G port from what I can see, but they don't mention speed for the Mini. Depending on the size of buffers on the intermediate switch(es), the speed mismatch can cause problems in some cases.
I've seen trouble with XBox360 as a Media Center Extender connecting to Win7 WMC. The Win7 PC has a good Intel NIC, is connected to the same gigabit switch as the Xbox, but the Xbox still occasionally spits out "network issue" errors which I can only surmise are somehow related to the speed mismatch (360 only has a 100Mb port).
TWC + TivoHD x2 (M-Card + TRs) > DirecTV MultiRoom > TWC WHDVR > TWC + 2x HDHomeRun Prime (M-Card + TRs) + WinMC + Xbox360 Extenders > Roamio Plus w/3TB, 2x Minis