Originally posted by P. Soo I want to replace the Tamron with a lens of similar focal length range that is pin sharp to the extent possible.
There's normally going to be a tradeoff between focal range and quality. A zoom lens with a long focal range is not going to be pin sharp all the way through it's range. If you want resolution throughout the lens, you're going to have settle for a lens with a narrower focal range.
When I owned the DA 12-24, my companion zoom was the A 35-105, an old manual focus lens with decent sharpness edge to edge and across its focal range (at f8). I've since tried the F 35-70 and the FA 28-105 f3.2-4.5. The 35-70 is similar to the A 35-105 except it features AF and is a lot smaller and lighter. Like the A 35-105, it features good resolution across it's range and is reasonably sharp edge to edge at f8. The FA 28-105 is an inconsistent performer, with serious border to border resolution problems at the wide end, but a really good performer in the 45 to 90 range (though not quite as good as the 12-24). The best Pentax companion zoom to the DA 12-24, however, may be the FA 24-90. I have no first-hand experience with that lens and can't say exactly what it's foibles and weaknesses might be, but images I've seen from it at the Pentax online gallery are stunning, the best I've seen from any non-star FA zoom.
If you need range beyond ~100mm the best of the non-star Pentax telephoto AF zooms would appear to be the DA 55-300 and the F 70-210.