I'm not sure if Mike has tinkered with A123's yet and I'm just converting my models, but I'll tell you that I have a 33% Edge with two 8411's in the elevators. I just changed out the Duralite Li-Ion/regulators for dual 2300mah A123 packs. I've not flown it yet, but I had no difficulties setting up the model and did not notice any eratic servo behavior while setting things.

The A123's do have a 6.6 volt nominal voltage, but that is less than a fully charged 5 cell NiCD or NiMH. I only have one model converted to A123's that has a servo which is not very happy and it's an analog from a different manufacturer. But it was also having the same difficulty with the freshly charged 6 volt NiMH that was replaced by the A123.

Maybe Mike can address the issue from the servo performance/requirements side but IIRC, Danny Snyder who used to run the JR manufacturer support forum in the radio section, said that while the official recommendation was for no more than 6 volts, there hadn't really been any issues with slightly higher voltages.