Hi - have been following this thread with interest - several things I can/should point out that would be of interest to HP Thin client users/owners.
a) The t620/620 Plus Thin clients can be obtained fairly cheaply on eBay (I should know, I used to manage a bunch of GT7725s as part of my job)
b) The power brick used by the entire T-series thin clients are actually the same as the HP EliteBook series of notebook computers (the ones with the 90w adapters pre-packed) and their accessory docking stations - old 90w docking stations from the HP Elite series are not that difficult to get a hold of. The t-series thin client has a reputation for being powerful and quite power efficient which goes back to the t7720/gt7725s.
c) If you are not familiar with HP ThinPro (Debian or Ubuntu based), there is actually an VMWare evaluation disk image of ThinPro dating back to the ThinPro 3 period that you can play around with right here:
ftp://ftp.hp.com/pub/softpaq/sp49501-50000/sp49650.exe
Unpack the VM, get it running, switch to admin mode (password is root) and it should prompt you to setup the ThinPro OS - keep it a multi-protocol setup with the classic GUI. After the reboot switch to admin mode, and use ThinState to restore a later (stock/not VM only) image.
The images are all stored here:
ftp://ftp.hp.com/pub/tcdebian/images/
- the one downside I saw from the 620 Plus is that the PCIe slot onboard is only an x4 (physically x16), so if you need to drop in a dualport 10GbE card, that's a no-no.
d) All t610s and 620s have the ability to take an M.2 based Fiber ethernet adapter (Allied Telesys AT27 or 29), both in SC and LC forms. The same can be said about the t630/t730 successors (not the same cards, though)
e) If you need more *oooomph*, I highly recommend the HP t730 thin client instead. While the t620/620 plus are GX420CA/GX415GA or GX217GA based (all AMD Jaguar based), the t730s are RX427BB (AMD Piledriver based). It also comes with 8 PCIe lanes in the PCIe slot, so installing a Chelsio T420 or SolarFlare 5-series card is very feasible. Current prices on eBay are about $200/unit, which is very competitive with most fully loaded Intel NUCs out there. I actually have a t730 serve as the hypervisor for my home lab (with a corresponding write-up. My only pet peeve is that the HP BIOS does not allow the IOMMU to be exposed in ESXi, so no VMDirectPath support as far as I can tell for now). I have no doubt that you should be able to do some serious packet filtering even on 10GbE line speeds, especially if you have something like SolarFlare's openOnload hardware based TCP offloading in a modern low latency tweaked Linux kernel.