Originally posted by rawr For info, according to LensTip, the Tamron 70-200 tests very well optically with a 1.4 TC (teleconverter), better than some other 70-200's. It seems to do very well even with a Sigma TC, so that one might be a good one to hunt down in K-mount (if one exists).
Do be aware that the Sigma EX 2x teleconverter in PK mount, although it would seem to fit most, if not all of the OP's requirements, is designed for use with specific Sigma lenses. On many other lenses, including some later Sigma lenses, it simply will not fit (the optics get in the way!) or if it does it may not lock into position ... I've got one, bought for use with a Sigma 70-200 f/2.8 EX, which it is fine with, but it won't lock onto my Sigma 150-500mm and simply won't fit at all with a selection of other Tamron and Pentax autofocus lenses!
The OP may also care to consider that, with older teleconverters, there is no way for the camera to know that the focal length of the lens has changed, so a 70-200mm f/2.8 still presents itself to the camera as just that, so the Shake Reduction feature may not be as effective as one might expect.