I have a device (LEGO PowerFunctions IR receiver to be precise), which operates at 9V, and usually, it's powered with 6AAA batteries. Needless to say, 6AAA batteries fit in quite bulky battery box, so sometimes people just use 9V batteries for it, which, on the other hand - yields much lower performance in terms of capacity (and they're more expensive than AAAs or AAs).
So - how about using 2 AA batteries (NiMH rechargeable, so with voltage of 1.2) with step-up converter to put it up to 9V? Assume the ones capable of 2600mAh, and the efficiency of step-up at 80% level, let's do some calculations:
2.4V / 9V * 2600mAh * 80% = 554.66mAh
So it would seem it's better than the 9V NiMH batteries on the market can provide. What would be the other considerations that I miss?