There was a time when Pentax and Tokina co-designed a series of lenses (who actually did what - who knows), the 10-17, 12-24, 16-50 and the 50-135. Pentax sold the K mount, while Tokina sold the others. So, when Tokina came out with its 11-16, I was looking to acquire the 12-24. I did wait around some time to see if Pentax was going to come out with their version of the 11-16 and what the price would be. Well that was years ago, and I guess whatever agreement there was lapsed. Bottom line is that Pentax still has no 11-16/f2.8, which is a shame since the lens is really really wonderful.
Yes, it would be really nice for astro stuff as a fast lens does come in very handy there. Even thinking about Sigma's 18-35/f2.8 maybe in the future (but do I really need more WA lenses). Other than for night astro, I have been able to get along ok. I do a lot of low light landscape/cityscapes. There are times where something faster would certainly help - but for the most part, the tripod is your friend and longer shutter times is a cure. I'm around f8 most of the time anyway using ISO 80. The only time where depth of field does not matter is in astro.
For astro, what I have done in the past is to use the 31Ltd wide open and stitch. That does work to a degree - in particular where there is a landscape component in the frame. I do feel that there are some limitations - a single row. I doubt that you really want to try something more complex. There is also the O-GPS which helps a lot, but then again with anything static in the frame, it is going to blur (due to tracking). So, in the end everything is a compromise.