I've never heard of what you said before - can you cite a resource that makes this claim?
It sounds like bunk to me - yes a computer will use a little more power on startup - but not 15% or even 15 times more - that's a tough pill to swallow. (if they did use more, you'd hear the cooling fans screaming to keep things cool every time you rebooted)
The Mini will use more power waking from sleep then if it remains in a sleep state - but how much power the wake uses boils down to how long it's active, and whether you manually put it to sleep, or rely on a timer. Keeping the "sleep" counter as low as possible will reduce power consumption.