[cyberjunkie]

... looks like the 50-500 is a good fit for the K1. As a side note, just got an email reply from a tech rep at SIGMA ...
... Per their reply "All of our current lenses ship K-1 compatible."

I guess you should go for the Bigma if you want a brand new lens. Great 10x zoom with very interesting features and good image quality. It's unique, AFAIK... no other 10x tele zooms in PKAF mount.
If you want to save money, you can either try to score one of the rare latest version Bigma's for Pentax, or go for the previous version, a little easier to find.
Even cheaper, you could opt for a 120-400mm EX DG HSM, as I have done. I still have to test it properly, but I can already say that it's focusing quite fast in AFS. If you miss a few shots in AFC the problem is not with the lens
As I've already written, I'm fully satisfied by the firmware/bayonet upgrade, done quickly, for free, and with no questions asked.
I'm also very satisfied by my Sigma EX DG f/2.8 (penultimate version of a list of five different models, IIRC), which was software updated but needed no bayonet exchange (only the latest version needs it).
If you happen to have a lens that is included in Sigma's K-1 compatibility list, I'd suggest to check the ID with a software tool that allows to access the full content of the EXIF, check with Sigma if it has to be updated, and eventually send it. I don't know if I would ever ship the lens for the scratch alone, but I think it's always better to have the latest firmware.
I'm not so sure it's only a problem of overlapping ID's, why on earth the same lens can either work fine, have AF problems, or even brick the camera until the battery is removed?
Lenses with internal AF motors have become complex beasts

BTW, if AF is not an absolute must, and you need a 500mm, there are a few much cheaper primes. Unfortunately all those lenses were not made to be used handheld. The flexibility of modern times AF zooms is really amazing. I love vintage long teles but I can't deny zooms are sooo practical
With a good zoom range, apart from the four Sigma's, you can find second hand the Tokina AT-X Pro 80-400mm. Screw drive AF, of course. Compact and not bad at all, considering it's a film era lens.

Cheers

Paolo