I have without any issue what so ever used my P45+ and my fiancé's IQ 260 tethered through the thunderbolt adapter relying solely on the computer powering the back. I have done this repeatably, even without a battery in either back, and have not run into any problems.
I do keep charged batteries on had just in case, but have never needed them.
The only issue I have had with thunderbolt was when I needed to perform a field line calibration. The program that you need to use does not support thunderbolt and crashes half way through the calibration update.