Originally posted by k100d common sense should say it should be better because of the less ambitious range. and bonus is also in the lighter weight
throw that into google and check it out "site:forums.dpreview.com sigma 150-500"
That might make sense (less focal range) but from everything I've seen/read it does not perform better then the Bigma. The Bigma is an EX lens which is Sigma's higher end glass, while the 150-500 is the more consumer level lens. Not saying that it is a bad lens but I believe the Bigma is better in terms of IQ.
John
Edit: of course nobody has tried the new version of the Pentax mount HSM version of this lens so it is all speculation until that lens is available