Yeah there are a few "bullet time" demos for clusters of Pis, very cool. Does your application use network based synchronisation, or do you hardware trigger? Whenever I've built computer vision systems it's simple just to hardware trigger everything (no freerunning) and then you just readout the last frame. Almost all image sensors support external trigger inputs, but you need the breakout board to expose that pin (basically all OEM machine vision cameras provide trigger inputs as standard, but they cost more than a Pi+PiCam!).

At IceCube we have 5k sensors synchronised to within a few nanoseconds using GPS fanout and some clever clock distribution algorithms [0]. Cool stuff.

In theory if you use a bright enough flash, the absolute synchronisation of the cameras is less important. For example very high speed photography tends to rely on hardware flash synchronisation with the event. In this case you assume that background illumination is low enough that a many-ms exposure will be black and all your light comes from flash pulse which can easily be sub-ms. Provided the event falls within the camera exposure window, you don't care if there's some uncertainty. You can do (1) camera trigger, start exposure (2) delay to compensate for maximum expected jitter (3) flash triggers (4) brief delay and then stop exposure.

[0] https://www.phys.hawaii.edu/~idlab/taskAndSchedule/ARA/0810....