Hi Patriot,
I have been using an FA*300/4.5 extensively for the past couple of years, and it's a great lens in just about every way. I had a quick play with a DA*300/4 and loved it, but decided to stand pat with my FA*.
My impression of the DA* was that it might be a little sharper (from a pixel peeper's perspective), it focuses considerably closer, and has the obvious advantages of SDM focusing, weather sealing and Quick Shift focus adjustment. The tripod collar is nice, but I've mostly preferred hand-holding my FA, so in this type of use, the tripod collar kind of gets in the way for me, since it's not removeable (IIRC). . . The DA* is @ a 1/3 stop faster, but it's a bit heavier also. I doubt that you could fault either for build quality.
If the DA* were significantly better optically, if I wanted to shoot in the rain, and if I planned on using this lens primarily with a tripod, I would have bought it. As it is, since it's readily available, and will probably be discounted now that it's been out for a while, I saw no need to upgrade at this time.
As for use with a TC. The jury's still out, AFAIK. I have an older Tamron 1.4x AF TC which has the PZ contacts, so it is able to supply power to the SDM motor in a lens with this feature. The only lens I've tested with it is my DA*50-135, and it won't focus correctly with the TC. I didn't try using it with the DA*300. Pentax apparently has a 1.4x SDM TC coming out pretty soon, so this might be the ticket for the DA*300.
I use TCs a lot, with my favorite being the Pentax F 1.7x Auto Focusing Adaptor. My FA*300/4.5 is apparently sharp and contrasty enough wide open to allow for it's use despite the relatively slow max aperture, and this combo works well, even when the light's not optimal. I have to assume that from an optics/IQ standpoint the DA* will work well with quality TCs, once the SDM compatiblity issues are sorted out.
Since you apparently have a Tamron 1.4x TC, it would be worth trying it with a DA*300 to see if it focuses satisfactorily. If it does, then I'd go with the DA*. I'll assume that the F 1.7x AFA would also work, since it doesn't depend on the AF capability of the lens to AF, but these are pretty rare and expensive these days. . .
If you can get your hands on either of these lenses, I think that you'll be very pleased with their performance.
Scott
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