It would seem that both the 15mm and 21mm wide from Voigtlander seem to exhibit less distortion and less vignetting than one would expect from them.
I currently have the 21/4 Color Skopar, when I was trying to decide between the two, and hope to get the 15/4.5 in the near future.
With my own experience with the 21/4 and what I've been reading from people who have the 15mm, there is no reason to think you cannot have a high quality ultra wide for an inexpensive price.
Oh if you thru your feet in the shot, find a location where having the bottom smaller than the top would seem funky 😀.