Sorry, but my internet connection is intermittent and the tablet isn't suited for longer replies.
The whole 35mm vs smaller sensors discussion is always going to be about application. Larger sensors have better noise characteristics and shallower DOF. Great cinematographers use those strengths. Close ups of talent etc are usually shot with shallow DOF. Similarly, quite a few people use the cameras both topside and underwater. That's why there was such a big thing about the ½" sensors getting S35 DOF for topside.
Still, with RAW/ProRes capability, the smaller sensors like the 4/3ish BM cameras have made the ability to capture proper correct 10+ bit clips without banding and other issues of H.264 and MP4.
As for the DOF "loss" issue, it's a CoC perspective that is relevant. Similar to the MFT to APS to FF comparions, DOF doesn't change but perspective does per focal length. I believe this was in the white paper on the Metabones booster.
There has been many discussions on wetpixel and the good old uwphoto list about the differences between underwater and topside photo and filmmaking. I for one never thought of this as one of them. For me the distinction is drawn more between the type of footage you shoot. Why are the small chipped cameras like the Ex1 so popular with the documentary run and gun folks? You don't have to pay too much attention to critical focus and you'll get more keepers when things fly by fast...
A lot of average Joes are bying the pocket camera thinking its cool (RAW and all) but really don't understand what they are using it for... Most of them would be much better served with a real camcorder. They just ignore the advise of "right tool for the right job" tip because pro and RAW sounds better!
For underwater small sensor size is a no brainer because as Thani pointed out you don't bring a dedicated focus puller underwater and marine life don't react well to instructions. + some dome-port issues etc.. But sometimes you'll ignore the benefits to just be able to actually get enough light. (for those of us that almost never find them selves in clear tropical water.
I've looked a bit more at the Metabones and I do believe that it acts exactly the opposite of a teleconverter. Increases light transmission, increase of sharpness, no depth of field loss. Anyone disagree?
1. I've used the metabones many times and I never knew that! RTFM Drew!
2. Yes you get 1 ⅔ better T*, but DOF goes backwards with the 1.74x crop. So even if you can shoot at ISO 250 for an equivalent , depending on shot, one may still have to roll back F* to compensate for DOF. Not to mention S16 has diffraction issues which are exacerbated with the booster, since it is sharper. As mentioned above, a shot you could shoot @ f11 (T5.6 light), now has to be shot at f16/22, where MFT shows quite a bit of diffraction.
Forget the S35 part, I was jumping between pages and I kept on typing! Thought I sent the corrected post but obviously not, due to the slow internet.
I personally think the BMPCC is a great little B/C point and shoot camera, vs an A cam. The advantage of the S16 sensor is the deeper DOF. ISO800 has noisy. It also will work well with macro due to the size and with lights, the noise issues aren't important. It does make shooting non-moving critters through flat port at f4 or less for that dreamy background a lot less appealing.
1. The metabones tripod mount is removable.
2a Not sure what you're saying with that first sentence, but as you are effectively using more area of the lens to project a image on the sensor you might also conclude that you have more light hitting the sensor for any given f stop. Now this might produce a very shallow depth of field wide open, but that was never my point?
2b Not sure where this fits into a discussion about the pocket camera, but I would argue that you would get better low light capability with the 4K sensor than the 2.5K sensor all else being equal. i.e you resample 4K down to HD, which I think the majority of shooters are thinking. Now there is probably a penalty on the sensor due to global shutter with the 4k version so they (vs 2.5K EF mount) might come out around the same, but surely a BMCC MFT with a speed booster would give you more light.