It's not efficient at all - the huge protuberance is awkward, makes the phone wobble when laid "flat", and catches on pockets. Efficient would be making the enclosure thick enough to fully house the cameras while having a completely flat back. The extra space could drastically increase battery life. Who cares how "thin" a phone is?? (other than Jony Ive, whom we have to thank for so many awful designs)
It would be fascinating to see what that would deliver even just as an internal prototype. I wonder whether Apple has ever built such a beast as a test article. Even a non-working dummy accurate in dimensions and weight would be an interesting thing to play with.
The 15 Pro Max is 8.25mm thick and that includes the thickness of the display, the screen glass and the back plate so I'd guess the thickness of the battery itself is only about 4mm. The camera and lens bump adds about another 4mm so a phone as you envisage could double the battery thickness and more than double the battery volume because the extra 4mm battery thickness wouldn't need to reserve space for the PCB, connector, speakers etc. My guess is that you could get somewhere between 2.5x and 3x the battery capacity in such a phone.
I would be worried about the weight but if that could be offset somehow, maybe by finding lighter materials for the back plate, then sign me up!