So I just got the latest version of Apple's MacBook Pro, specifically the highest end configured 15 inch MacBook with the fasted processor available.
With all of the hype about how thunderbolt does not provide enough power to power the back, and that the P series backs do not charge the battery in the back, I was worried I would need to buy more batteries and keep batter track of how charged they are.
But before pulling out the wallet, I decided to test this on my own to see how well or not thunderbolt powers my back. I took out my Arca, through on my P45+ with no battery and put on my Roddie 55mm. I plugged the back into my computer and viola, it turned on and connected with no battery.
I set the ISO to 200, shutter speed 1 sec at f/11 and took a picture of my dim kitchen totally expecting the back to give me a error message. It did not; the image come through with no problem. So I dropped the ISO down to 50 and did an 8 sec exposure, thinking maybe a long exposure would be too much. No problem once again; the back worked perfectly fine.
I changed to my SK 35mm with the center filter and composed my angle with live view, think live view would probably be too much. Live worked fine and I took my time composing my angle. F/11, ISO 50, 35 seconds and no problems. I put on a polarizer and did a 2 minute exposure and once again, no problems. I unplugged my laptop and did it again, nothing changed, the back worked perfectly fine.
So it appears either I am really lucky, or the new MacBooks were built with a little more juice coming through thunderbolt.