If the power on time can be reduced, say by half or more, then there would be no need for the sleep mode. As it stands I really cannot see any value in the sleep mode other than locking the buttons - what other purpose does it server? - only of value to those who put the reader down (or pack it away) and pick it up again within 15 minutes. How many people do that on a regular occurrence? If you put it down for say 5 minutes, would you bother putting it into sleep mode? If it's just to lock the buttons, why not just leave the current page displayed for the 15 minutes - perhaps with a little lock symbol top right.
I'd suggest focusing more on reducing the power on time and doing away with the whole sleep thing - and bring back the cover as priority number 1
