If you take a look at the first post, Amir measured it in high gain. It looks like performance is not compromised even at max volume, so you should be fine. I also use it in high gain and I don't notice any problems.
I know this thread hasn't been very active, but I recently upgraded from the original Fiio Q1 (Mark I) to this Mark II model, and it's an amazing portable device. The main reason I upgraded was to have perfect channel matching with low impedance IEMs and portable headphones, as the JDS Labs Atom was giving me channel imbalance issues in that regard. The Q1 MK II has perfect channel balance even at the quietest volumes. I'm even using it with the Sennheiser HD 600 and 650, although it doesn't get loud enough with classical and other quiet recordings. Great for everything else though, and plenty loud enough with the Focal Elex. I'm considering getting a 2.5mm balanced cable for the Sennheisers to take advantage of that extra power.