Hi ruemiser,
I'm primarily a long tele shooter (birds), and still have a very old version of the Tamron. I have to admit to not shooting any of the more recent Sigmas, the last one I tried was probably 6-7 years ago. After shooting a premium 300 prime (A* 300 f4), I gave up on the xx-300 consumer grade zooms and now most often shoot FA* 300s, both the f4.5 and f2.8, with or without TCs, so I'm admittedly pretty much of a lens snob when it comes to lenses in this FL range.
About a year ago, I bought a used DA 55-300 for its light weight and convenience as a walk around lens, and feel it's pretty easily the best of the AF xx-300 consumer zooms for Pentax from a telephoto performance standpoint.
It can't compete with the FA* primes, but it's plenty sharp wide open, and stopped down to f8, I really have nothing to complain about.
Here's an example at f8, 300 mm
I also shoot a Q for super tele reach with adapted lenses -- the Q is a real test of optical quality, and the DA 55-300 is proving to be a surprisingly strong candidate for use with the Q handheld now that Pentax has added Focus Peaking to aid Manual Focus, and SR to make handholding at the extreme FL EQ (1674mm in 35mm terms). In a short test (the FW update came as winter was settling in where I live), I found that I could handhold this combination pretty reliably down to about 1/125, but at 1/400, it really started to show its potential. This is full frame from @ 15 feet, lens wide open at f5.8, 1/400, ISO 125, slightly sharpened and downsized for posting. This is not meant to be an endorsement of the Q as a long lens solution, (though I don't shy away from doing so
). I felt this is a good way to illustrate the optical quality potential of this lens.
The full post is here:
https://www.pentaxforums.com/forums/pentax-q/204091-q-fw-v1-1-da-55-300-handheld.html
The Pentax is pricier, but worth it (for my purposes).
As far as "macro" goes, it's good to get some perspective on how much magnification 1:2 gives you. The sensor in a Pentax DSLR is about 24mm wide, and a US Quarter is an easy reference to that length. With a 1:2 mag ratio, two quarters side-by-side will fill the frame of the shot horizontally. This is good for butterflies and mid small flowers, but will not give you those eye popping closeups of small insects. As has been mentioned, an achromatic closeup like the Raynox DCR 250 will give you some very high magnification with one of these tele zooms, and is a good, relatively inexpensive way to introduce yourself to the macro world.
Scott