[davidmanze]

I can confirm that the Tamron 150-600mm's VC struggles beyond 500mm. First I thought it was a defect but then others reported this as well. The Sigma 150-600mm S has no issues here.

 Tamron have a firmware update which is supposed to cure the VC  problem when panning, all it does is switch off the VC, so far I haven't sent it in to be upgraded.

 I have the Tammy  17-50mm non VC version and have used it on the Pentax K20/K5/K3 with no SR problems, great lens, very sharp. 

   One strange problem however, which I did post here some years back. Wide open to F5 the AF is spot on, when you stop down to F5.6 it front focuses. Put the camera into AF-C half-press the shutter button, and with the rear wheel, change aperture from F5-5.6 and it refocuses and becomes front focused. but only at less than 3-4 meters.I posted my findings on DPreview and had six to eight replies from other users who found the same thing. Strange!