I have no experience with the Sigma 50-500, but I have both the Sigma 150-500 and Pentax 60-250. If I assume the 50-500 and 150-500 are simmilar in IQ then I agree with you about the quality of the pictures the Sigma's take. They lack 'punch'. I'm not sure if it is micro contrast, colour saturation or just inferior galss, but I can tell the difference between a sigma shot and a Pentax shot immediately.
I only use the Sigma when I need the reach, the 60-250 is far and away a better lens in every sense. I posted a few sample pics from the DA* 60-250 here
https://www.pentaxforums.com/forums/lens-clubs/73997-da-club-60.html#post1865935