Maidened my EFX on Sept 6th, putting three Zippy Compact 4S 2200 35C packs through it on the stock 5x5 prop. It went pretty well - not quite 100mph performance I don't think, but close enough for me, at least for the moment. Some observations...
Mixed throttle flights of 4 mins seem well within the packs capacity, even though it is losing a bit of punch at the end; final resting voltages were 3.8-3.85V/cell in all three packs. As I have posted before two of the three are exhibiting high internal resistance (20-30mOhms per cell at a nominal 3.85V/cell) so I don't anticipate they will last very long, but I have decided to keep using them as an experiment rather than returning them to HK - they were only ~£12 each so the cost of postage would be pretty high compared to their value.
With this battery CG is at ~55mm, clearly at the aftmost range. Inverted as you would expect needs little down, and the elevator is sitting neutral so unlike some others I don't think there is an incidence issue with this model; it is just designed to fly at 55mm back from the LE, not 50mm.
Mine seem to roll left slightly; I can't see a warp, but will double check carefully before the next flight and verify I don't have one wing heavier than the other.
High rate was way too sensitive on aileron and elevator for my taste at the recommended 30% expo; have