I am a late-comer to this thread, but may I ask why you want a
fast ultra-wide?
If it is to get shallow DOF, I would suggest that while the idea sounds cool, the reality is less so. Ultra-wide angle lenses (fisheye and rectilinear) are notoriously difficult to focus. I use a Katz Eye with my Zenitar and without its focus aids I would be lost. The same issue exists for focus confirm using the AF system.
If, on the other hand, you are wanting to do low-light shooting and need the faster glass for that, see paragraph above and double the emphasis. A better bet is to get a camera with excellent high ISO. But don't wide-angles have super DOF? Yes, they do, but there are still limits. The best you can do with low light is zone focus. There is no way to do otherwise in dim light.
Steve
P.S. In regards to the Zenitar, here is a link to images (both APS-C and 35mm film) from my Flickr account taken with that lens.
http://www.flickr.com/search/?w=28796087@N02&q=zenitar