I had this lens in Nikon mount. It had Sigma's ring-type focus motor. I don't believe the 30mm for Pentax does. Too bad because it was great! The build quality and heft were very comfortable, and the focus was fast and sure. To be honest, I've left Nikon but probably won't be buying this in non-hsm for Pentax. I haven't used it, but I suspect the heavy elements might be slower to focus (louder for sure). The 7 is supposed to have faster body-driven focus. I simply can't say. That depends on your desire for a fast and powerful hsm motor.
As for the image quality I was very pleased. This lens is great for people inside, evenings, etc. GREAT BOKEH--and I'm taking quality not just "out of focus". I love the image. Great for portraits. I went on vacation to NYC and did some street shooting and was nothing but pleased. Soft corners don't matter to me unless I was doing very technical work, which to be honest I don't really. Center sharpness and overall quality made it a "want to use" lens. Pentax is said to have a fast 30 somewhere on the horizon (how distant please?) I would also look at the fa35. Bokeh isn't as good... I would say "imo", but it just isn't (imo
I haven't checked prices lately. Hopefully other people suggest some good options. I'm liking the size and macro of the 35ltd, though it's 2 stops slower than the sigma. If your shutter speed can stand to slow down a bit I'm sure 2.8 on the ltd would look great. A little extra depth of field also helps a bit with focus.
Most every lens available is great for artful photography. It's often the one you're excited to use that makes the best pictures.
Good luck!
chris