For this post I did a direct comparison between my Olympus 7-14mm f/2.8 zoom at 9mm and my Samyang 14mm f/2.8 (Nikon mount) fitted to my Panasonic GX8 via a Metabones 0.64x Speed Booster. Both lenses were then approximating a 'full-frame' / 35mm sized sensor at 18mm. The idea being to see if there was any significant difference between the two. I tested each lens at f/2.8 and f/8 at ISO 100 using the same camera. I picked a very contrasty scene as well to see how each lens handled that. The results were pretty much as I expected from my previous experiences working with both.
1. The Samyang + Metabones does have that 1 stop light advantage. However, there was something I discovered. That combination overexposed slightly, whereas the Olympus zoom got the exposure spot on. Now this is something that I have noticed before. The speed booster does have a tendency to do this on the GX8. It's not by much, just 1/3 to 1/2 a stop, but it is there. I usually compensate for it when I use it.
2. In terms of image sharpness, it was virtually impossible to tell any difference, either from the raw or jpg. files. Both lenses, being ultra-wide, aren't the sharpest you can get anyway, but are still perfectly acceptable for any kind of reproduction. In fact that's a bit of an understatement. They are in fact somewhat more than acceptable.
3. I couldn't detect any difference in terms of distortion, CA etc. that would effect reproduction.
And this is often the case with adapted lenses, that they can be perfect in certain situations, but sometimes for general use they can be somewhat limited. With the IBIS of the GX8 the MF Samyang pried becomes more useful, but then the zoom has that too.
i see an advantage for what I do with the 14mm and in many recent cases it proved to be just what I needed. With the weather and the upcoming Christmas holiday I've been shooting a lot indoors recently with wide apertures and high(er) ISO's and to keep the quality of what I'm shooting as high as possible, that extra stop from the speed booster has proved invaluable. While I have other excellent lowlight alternatives, including my Leica Q, there are often cases when that isn't wide enough and the Samyang earns it's keep.
It's also worth remembering that certain lens mounts have the added benefit of being able to be used on various cameras. There are some lenses that have been released in a m4/3 lens mount, but for me the Nikon mount version is always the most useful. I can use my Samyang 14mm on my Nikon D750, Canon 5Ds and my m4/3 cameras. And If I return to Fuji X or Sony e-mount then I can use it on those as well. And Samyang are certainly promoting themselves these days as the provider of cheap(er) lenses for video. They now have some sets of fast lenses in cased kits, providing options for film makers to use on a variety of cameras.
And combined with that extra stop via the speed booster, that is the advantage of advantage of adapted lenses, the possibility to use a lens on different lenses. And there is no doubt that versatility has it's benefits.