Yes, and it's not clear to me (yet) which are the outstanding lenses. The 31mm's focal length is an obvious choice for me, and it's a relatively fast lens. But to be honest, I'm probably perfectly happy with just the 31mm. Though I'll probably also get the 55-200mm for my kids sports. And probably the standard WR kit lens just to have a WR kit.
My intuition is to keep the K5 and learn it.
Do that. I had a terrible time when I first got mine. I had previously had a K200D, a Kx and a Kr. The K200D was the last to be sold, because I loved it to bits. ISO only went to 1600 but I didnt care, because I shot at 100 whenever I could, anyway. It was an entry level camera with many of the pro features you would only find on more expensive units (weather sealing, top LCD and more). It did not have live view which I used to bemoan, but now I have it, I rarely use it anyway. In any case, the K-5 was a huge step up from that, and I think its going to be a huge step up from any camera which isnt in the same category.
I was very disappointed at first, I wasn't producing the goods, and it wasnt because I thought the camera was no good, I just thought perhaps my skills were well below the camera's capacity... in the end, it was, quite simply, a matter of learning and practicing, and once I had no K200D to fall back on, it was that, or sell... and I didnt want to sell. I'm really glad I didnt.
Lenses which are supposed to be outstanding:
12-24 f/4
your 31
FA43 and 77 (though many prefer the DA 21/40/70 combo)
Best thing to do is visit pentaxforums.com and the lens database, and then go to the pentaxphotogallery.com and have a look at the photographs taken with the K5 and the particular lens you might be interested in. There's some wonderful stuff there (and also some crap) and you'll get a good idea of whats best for your particular intention. Ignore photozone.de because they play down nearly all Pentax lenses, unfairly I believe: its not all in the numbers.
If you have strong hands and don't mind a bit of weight, the DA* lenses (I've got the 55 on my wishlist) and also some of the sigma lenses are worth a look.