Daniel, the Sigma 17-50 is only going to give you a normal to long tele lens, no wide shots, and any focal length past 20mm is too hard to hand hold without lens stabilization. I went the Speed Booster, large lens route on my Pocket camera, but discovered this setup was counter to the entire concept of a small camera setup like the Pocket camera. The Sigma Zoom is also probably not parfocal either, which is another issue with still camera zooms, and will require refocus every time you change (zoom) the focal length. The “normal focal length on the Pocket is 17.5mm. With a Metabones BMPCC SB you will get a 10-29 however, but with a heavy setup that is best left on a tripod, and will need additional support.
You will need the Canon EF version to get OIS, and add a $600 Speed Booster to a $300 lens, adding up to $900 to make it useable on the Pocket Camera. The Nikon version is less, so is the BMPCC/Nikon SB, $300-400, but no OIS, auto iris or push to focus via the camera. The Sigma is large, with a 77mm front and weighs almost 2 lbs.
I ended up going back to native MFT lenses, like the Panasonic Leica primes and the new PL 12-60f/2.8-4 Zoom, which has excellent IQ, is parfocal and was designed for videography in addition to still photography. The auto focus is fast, and manual focus by wire is almost like using a real manual focus, very responsive. The servos are dead quiet and will not record any noise.
Another larger, and a little heavier option is the Olympus 12-40 f/2.8 zoom and the smaller and lighter Panasonic new version of the 12-35 which also has OS, constant f/2.8 like the Oly 12-40, but smaller than the PL or Oly. A 12-35 or 12-40/60 Zoom range is much more useful on the camera than a 17-70 (I also have the 17-70 Angénieux Zoom, and only us it for outdoor work, when I need the longer focal lengths, like nature shooting).
Many early Pocket camera shooters started out with the Panny 12-35 Zoom, due to is compact size and light weight, it’s excellent IQ and ease of use. Using native MFT lenses, eliminates the awkward adapters and Speed Boosters. Which need a rail setup or other means of supporting them in the MFT mount to eliminate lens mount play, which translates to yiur image jumping when you move the lens focus or zoom.
Native MFT lenses fit the camera tighter and do not have this issue, as their focus is not dampened as much as manual lenses.
Food for thought...
Cheers