The people on the German forum don't know anything about the US units, no help from there I'm afraid. But we had already several very good posting here which help to make a decision.
The wiring is supposed to be the same comparing the EU and the US unit. The faceplates are different between the model specific units and the 4B0 unit is the only one which supports a separate speed signal. Any other unit (8E0, 8H0, 8P0) supports only speed via CAN. There was one posting here saying that the 4B0 can be programmed to look for speed either on CAN or the separate speed wire (and both actually work if the car supports both). There was also one posting here which confirmed that the other units can be programmed to expect speed of the separate wire (which doesn't exist on those models and therefore makes the navi not working to well). One thing has been confirmed several times: you cannot get an 8E0, 8H0 or 8P0 unit to look for speed over the separate wire, these models only accept speed over CAN.
The other differences concern SAT radio (does not work with any EU unit yet), IceLink iPod and Dietz video adapters (not confirmed working with the US unit yet) and the difficulties with some very small streets and the modified US map in the EU unit.
And both the EU and US units have an issue with keeping the time from the instrument cluster and the RNS-E in synch (concerns for example pre-2001/2002 cars like the A4 B5 or early A6 C5). Based on what I read so far, newer (2002+) A6 C5 instruments clusters properly support the RNS-E and may fit in the A4 B5. There is no special S4/RS4 instruments though, you can't keep both.
Good luck...