I have the Sigma 150-500 F6.3. I get great photos with it, but a little slow in low light but still a very good lens. About 5weeks ago I was able to purchase a used Sigma 500 f 4.5! This lens costs alot & is very heavy but I am getting very sharp photos. As far as Sigma lens for pentax mount they work great for me. Another Sigma I have & is a very good lens is the 300 F2.8. Put the 1.4 tele extender on it & you have a 420mm f 3.2!
Let me know what you decide to go with.