This was an EOS M [EF-M] Mount Lens so no speedbooster used here. 2.5K has a crop of around ~2.9x so very useable with 16mm format lens. Viltrox speedbooster is absolutely fine, no issues here at all. Use it with my Panasonic cameras too, no issues at all. I don't see any degradation, and nobody watching my videos has ever mentioned any degradation. If I never told you, you probably would notice the difference between the Viltrox and the Metabones version both of which I own.
With 2.5k Mode, yes, the wider the lens the better. 35mm on a speedbooster and less is better. Good options are Samyang 12mm, 14mm, 16mm or Tokina 11-16/Sigma 18-35mm which will work perfectly with 2.5K RAW...Insane, in fact.