Also, what about the size of the front element? Optical vignetting will deem to be massive if the lens is for ff!
Uh, you might want to take a look at the vignetting of... *checks notes*... literally every RF lens released to date. Every lens is about a stop darker than the nearest EF equivalent. Canon, regardless of whether anyone thinks they're right or not to do so, have decided vignetting is one of the aspects of a lens they do not need to correct optically any more.
I'm assuming that as a prime lens that it would have less barrel distortion than an equivalent zoom.
Same deal here. While the distortion on RF primes hasn't been so different to EF as the vignetting has been, in general they still requiring a little more correction. I can't imagine, for one second, that Canon would make this small 16mm have less distortion than, for example, the 14-35mm f/4L which they're charging about, what, six times the price for? Though, yes, all other things being equal you would expect a prime to have less distortion than a zoom, considering the price and size they've gotten this lens down to and how loose they've been with optical corrections so far, the smart money is betting on this lens being practically fisheye uncorrected.
For reference, Fuji's compact 16mm f/2.8 also has heavy barrel distortion which relies on mandatory opcode corrections, and that lens only has to generate an APS-C image circle. The old EF 20mm f/2.8 has strong wavy distortion, and that's a much larger lens and a narrower angle of view.