Guys who can share sample shoots with Canon EF-S 17-55mm F2.8 IS USM and Sigma 17-50mm f/2.8 EX DC OS HSM on Blackmagic cameras?
After the first 3 shots, it's the Sigma 17-50mm with the Viltrox speedbooster:
I also own the Canon 17-55mm and the Tamron 17-50mm.
I honestly dislike all 3 lenses: Tamron's VC and Sigma's stabilization are abysmal, and while Canon fairs better, it has a very annoying mechanical design flaw: if you move the camera downwards, the zoom lens will zoom-in all by itself (essentially, the zooming mechanism responds to gravity). Very annoying. Other zoom lenses, even heavier lenses, don't do that.
Also note, the Canon 17-55mm does not fit speedboosters, because it's an EF-S lens, not a plain EF.
As for Tamron: I have two of these same 17-50mm lenses and BOTH have the same problem: the front filter ring becomes loose after a couple of years. And while this is not an issue for photography, it's a pain for video, because the filter ring does a clang-clang noise when moving the camera around, that the camera's microphone picks up.
If you're looking into good-enough stabilization with an EF lens, then you're going to have a bad time. Canon developed a version that works well-enough with video only in 2015 onwards, so apart from the EF-M 15-45mm (for EOS-M) and a couple of EF lenses around that time, you won't find good video EF stabilization lenses. And it's not about USM support either: it's about being specifically optimized for video, and not just for motor noise. That EOS-M kit lens for example, is probably the best stabilized EF lens ever released (the EF-M is not mount-compatible, but it's the same underlying architecture as EF).
You'll have to go mirrorless to find good implementations (e.g. the Sony 18-105, 28-70, some M4/3 lenses, and all stabilized R lenses). You won't get the same stabilization performance with EF lenses, particularly with third party lenses. Tamron has micro-vibrations, and Sigma does weird things at times: steady, steady, steady, jerk, steady, steady, jerk... (all by itself).
Bottomline: get the Sigma, disable its stabilization, and use it with a gimbal.