My OBi202 DHCP server currently has 11 devices registered. The main problem I see is that the DHCP reservation table only has 20 entries and I've already maxed that out. I don't see any problems with this number of clients, but then I wouldn't expect to.
I've not seen any WAN slowdowns that I can't explain by issues in my ISP - I'm limited to 3Mbps upload and 1Mbps download, and I can and do hit those limits. My ISP is also working via a radio link and that does show slow downs (e.g. it's raining heavily at the moment and I'm seeing about a factor of 10 reduction in speed.)
I don't know about the UPnP support. The QoS settings on the OBi 202 seem weird and they are difficult to change (the remote, Obitalk, interface doesn't seem to work at all). There might be some major issues here, but I've been using Blink on a PC and got satisfactory call quality - I have tweaked the QoS settings though.
I think maybe the 202 isn't doing any UPnP; that might be a major issue for you (and, indeed, everyone.)
I don't think the power supply is an issue - IRC it's just a standard output voltage (IRC 5) and I think it is a switching supply (again, though, I can't remember).
I've also discovered that UK numbers work just fine when served from the US - I've got a UK number that is handled in Seattle (on the west coast of the US) and I get fine call quality whether I'm receiving in the US or the UK. So I think if you have an OBi202 (or something equivalent) you can use whatever provider is cheapest, regardless of where they are in the world.
John Bowler