ive been looking at the Tamron 70-200 f2.8 also the sigma 70-200 f2.8 apo 11 HSM.
the example images Ive seen so far are that the Tamron produces better contrast with improved saturated colours.
Ive also looked at the 60-250 Pentax f4 , even though its slower it does to my eye produce a better image in both contrast and saturation ,
IQ of all the lenses seem to be on par with each other....but id say Pentax has the edge, followed by the tamron then sigma....Pentax only problem is the price.
this is based on examples of images produced and not my own first hand experience .