On battery technology....
Point 1) There is far better battery technology in labs than Canon, Nikon, or Sony is using in their cameras.....
Point 2) Until these new technologies can be produced reliably and cheaply, they will not be mass-marketed.... nobody is going to buy a $10,000 battery for their $1000 camera.....
Point 3) Once one of these newer and superior battery technologies reaches the point where they can be economically and reliably used in cameras, EVERYONE will start using them, so the relative battery life between brands will remain unchanged.
On comparing battery life between mirrorless and mirrored cameras.....
Point A) What are the power requirements between each for focusing camera lenses and how does it compare?
Point B) Same question, only for IS systems......
Point C) When using a mirrored camera in "live view", the mirror is energized and that is a major power drain...
Point D) The rear LCD panel on a mirrored camera and a mirrorless camera SHOULD consume the same amount of power.
Point E) Since mirrored cameras do not have an EVF, that is another power drain that mirrorless have that mirrored do not have, although this is partly mitigated by the info display on viewfinders and shoulder displays on the mirrored cameras.
Point F) A comparison where one is using an EVF and the other is using an LCD panel display plus holding a mirror up is not an even comparison.
Point G) In general, mirrorless cameras have more battery life than their mirrored cousins when working in live view mode.
Point H) In general, mirrored cameras have more battery life than their mirrorless cousins when the mirrored camera is NOT in live view mode.
Point I) The advantage seen in point G is roughly proportional to the larger size of battery found in the mirrored cameras.
In general, the level of technology in mirrored and mirrorless camera is about the same..... the big difference is the mechanical mirror. It seems that it takes a lot of energy to hold that mirror open. Let's say Canon took the 7D2, added in an EVF, and took out the mirror..... Since it appears that mirrorrs take a lot more energy to hold open than it takes to run an EVF, it would be a safe assumption that a mirrorless 7D2 would get a lot better battery life than a mirrored 7D2 in Live-view....
How would it compare to a Sony mirrorless for images per watt of battery power? Probably worse as it has a lot more computing power.... but a mirrorless rebel would probably be about the same. Ultimately, nobody knows as there are far to many variables that none of us know about to make any kind of prediction, but regardless if the number is 300 or if it is 3000, most of us will carry around a spare battery (or more) so in the end, does it really matter?