It's actually 3U since the 2U of 40 pis will need almost an entire 1U 48 port PoE switch instead of plugging into the TOR. The switch will use 35-100W for itsel

It's actually 3U since the 2U of 40 pis will need almost an entire 1U 48 port PoE switch instead of plugging into the TOR. The switch will use 35-100W for itself depending on features and conversion losses. If each pi uses more than 8-9W or so under load, then you might actually need a second PoE switch. If you are building full racks, it probably makes more sense to use ordinary systems, but if you want to have a lot of actual hardware isolation at a smaller scale, it could make sense. In some colos, they don't give you enough power to fill up your racks, so the low energy density wouldn't be such a bummer there.