If I was looking for 17-18mm, settling for 20mm would be quite a compromise. I understand the problem in that almost all full frame UWA lenses were designed well before the digital era - and design capabilities have improved considerably in this area. Most of the development went into zooms, and Pentax isn't the only brand limited when it comes to FF and UWA prime choices.
IMO, the Tokina 17mm is a better option than the Pentax 20mm not only because the price is typically reasonable, but the FoV is dramatically more impressive. I had the AF version on a Canon 5D. Here is an example showing its strengths across the frame:
A-List Images - James Robins - Powered by Phanfare
The Tokina is available only in MF for Pentax, and the reviews here on PF are fairly positive. The optical design is the same on MF and the two AF versions that continued until 2005. Changes were made in barrel, coatings and aperture design, but the relatively high overall optical qualities of all three versions were close. (Tokina wasn't ever the best in coatings, so the main fault with the lens will show when allowing direct light to hit the front element.)
Unfortunately, the better alternative choices covering UWA in FF are zooms, but all are quite large. I have the Sigma 12-24 that I intend to sell (purchased on a whim before deciding not to upgrade to the K-1), and that lens is relatively small for a zoom and quite good starting around 15mm assuming a decently centered copy - but nothing like a compact prime in size, of course.