First off, I don't have any experience with Tamron lenses. However, I do own a lot of lenses, a few of which are third party like Tamron.
It's my experience that if you buy a lens because it looks comparable to the camera brand's own model with similar specs but at 1/2 or 2/3 the price, you'll end up disappointed. You're just going to end up wishing you'd saved the money you spent on the third party lens and instead put it into the top of the line lens. Unfortunately, my experience is only with Nikon vs. third party lenses, and so I can't speak for the quality of Pentax lenses.
Basically I'm not really much help at all. But that's my two cents. I will buy nothing but Nikon glass for my Nikon body from now on, for what it's worth.