Looking at teardowns, like the one at ifixit [1], the touch id sensor seems to be a pretty standard imaging sensor that heads to an NXP chip. I'd be willing to bet that the encryption of the print happens on the nxp chip instead of the imager, so if the NSA/whoever were doing a "repair", they'd probably just put an MiTM chip on that insecure path for later playback. Against a state actor, Touch ID is a triviality.

[1] https://www.ifixit.com/Teardown/iPhone+5s+Teardown/17383