I just got my RF16 copy and wanted to share my first thoughts. And please hold on to your butts, strap in, as this will propably (not) shock you:
You can NOT (!) turn of the distortion correction, regardless whether you're shooting raw or jpg. You can trick your r-whatever into showing the whole picture (literally) when you slightly unscrew it from the mount, forcing the now not properly coupled lens to to show the whole "mess". It gets a lot wider, a lot more distorted, but interestingly does not show too dark corners. At least at infinity, that is. At the closest focussing distance it shows basically black corner-extremeties gets even more distortion and a much (!) narrower view. This lens focus breathes athletically.
Image quality, from what I could test so far, is okay-ish; the corrected image is nearly flat and shows basically no distortion, center is sharp, corners are unsurpisingly less so at f/2.8 but do get better at smaller apertures. I would guess that my EF 16-35 f4 L is probably a whole lot better (when compared at the same apertures).
But: It is tiny. And light. And to be honest: As a freakishly small lens for strolling around, hiking without too much gear or even cycling (for which I bought it) it ain't too bad a deal. At least so far.
