I have never used the 15/3.5 but I own the 20/2.8 and I find it one of my best lenses, on film as well as on digital. Maybe you should reconsider your judgment on this lens. I have also tried and returned the 16-45/4 because of the ugly cheap-digital-P&S look and omnipresent purple fringing.
I have tried quite a few wide lenses (mostly primes -- I don't like zooms so much -- some very old) on my K10d, and these are the ones I liked the most:
- Super-Takumar 20/4.5 (quite good on K10d, but the FA 20/2.8 is much better; a little sensitive to flare; the borders are not so good on full frame)
- Super-Takumar 24/3.5 (nice lens, a little sensitive to flare; the SMC version should be better regarding flare)
- Tamron Adaptall SP 17/3.5 (sharp enough stopped down to 5.6 but some purple fringing and very sensitive to flare)
- Tamron Adaptall SP 24-48/3.5-3.8 (nice compact lens)
- Tamron Adaptall 24/2.5 (a little soft wide open, maybe mine has a centering defect)
- Sigma 20/1.8 (soft and low contrast wide open, becomes quite good around f/4.0)
- Sigma 24/1.8 (much better than the 20/1.8, even wide open; would be perfect if it weren't huge)
- Pentax FA 24-90/3.5-4.5 (very good all purpose zoom, on digital as well as on film)
As I also have a Canon 1Ds, now I try to focus (sorry, pun intended) on M42 and Tamron Adaptall lenses, because I can use them on the Canon as well as on the K10d. I think you are right about full frame coverage: it's only a matter of time before Pentax produces some full frame DSLR (if it stays afloat long enough). If Pentax fails in this regard, you might want to see what the competitors have to offer
Cheers!