If possible, better battery life on the E11 would be a great feature people would appreciate. I know the Arrow can get up to 50 hours of playback, the RSA Shadow gets 72 hours, and the iBasso T3 gets 38 hours. If the stated 10 hours is really true, I can see it being a big deal breaker especially since this is targeted as a portable device.
I know the Arrow uses a 1200mAh battery.
Also, please give the mp3 player you're developing at least 16 GB of internal storage plus a microSD card slot. That way people don't have to bring around multiple microSD cards which they will probably lose.
The way Robert is able to make Arrow run for so long is because he implements an auto voltage/current limiting circuit that keep the opamp running on lower voltage and consume less current when extra output power is not needed. I would imagine putting such an extra circuit will only add complexity to the design as well as parts and manufacturing cost. If it runs on high current mode all the time, then Arrow will still have a short battery life. As James already pointed out, E11 is going to be cheaper than E7, so you can really pick one way or the other using a fixed voltage/current supply. Although a manual switch would probably be more feasible, sort of like those on 3MOVE / StepDance.