Originally posted by TwoUptons reeftool, I think you misunderstand.
My Tamron 70-300 significantly predates the K1. I bought it for the K10, and I like it because it is a full frame lens that takes decent pictures, and as a bonus, it has an aperture ring, so it works a treat on my old film cameras if I ever play with that whimsy.
I've actually been pleasantly surprised with the results on the K1. Like you, I wasn't expecting much, but after spending some time with it, I feel it works far better than it should considering what I paid for it. And the high-iso capability of the K1 lets me stop down a bit where the purple fringing isn't so bad. The relatively low pixel density of the K1 compared to a K3, for example, helps here.
I'm actually rather annoyed at Pentax for not having a full frame version of the 55-300 out by now. By all reports it is superior to the Tamron except the APS-C image circle. The brand new DA version is $399. Surely for $100 they could do something about the image circle...
Other alternatives are the DFA 70-200 f2.8 and similar takes from other manufacturers, but I don't need the speed, and I don't want the weight. I want a decent travel zoom that's relatively light and WR, which those are not. Or I could spend $1300 on a 60-250 and void the warranty removing the baffle and still have a heavy lens. If I had one already, I might risk it, but not at this point. And if they upgraded that lens to cover full frame, I would consider it, but right now it doesn't, and I don't want to spend that kind of money on a half-measure, even if I was OK with the size.
I would certainly consider an improved 70-300 from Tamron or Sigma (or anyone else, really). If there were to be lenses those manufacturers supported in K-mount, I would expect those would be the ones. It's an obvious upgrade from a single lens kit with a much broader appeal than high end stuff or niche primes. My only concern there would be if the lenses were actually improved...
At the end of the day, this is about matching what I need from my kit. The Tamron is my best, albeit imperfect, match at this point in the market. I hope that changes, but I'm not in a great hurry...
-Eric
Sorry but I took your post to imply that the Tamron 70-300 was a suitable FF telephoto solution to K1 owners. If you only use it outdoors on bright sunny days shooting at f/11 on a tripod, it may well be suitable. I'm a long time Sigma 70-300 user and my lens has generally been a good performer for me. I've used it since day one on my K10D. Unfortunately, Tamron and Sigma appear to have abandoned the K mount. I also have a Sigma 150-500 that is FF and I have seen a few decent shots posted from the K1 with that lens and it's Bigma sister, the 50-500.
You're right, Pentax really needs to step up and get a reasonably priced telephoto that covers the FF circle if they are serious about competing with the big dogs. That said, while in the future I may pick up a K1, I will also be holding on to an APS-C body where you have an advantage with a telephoto. I shoot a lot of sporting events, mostly motorcycle racing, where I can't get too close. I have been extremely satisfied with my K5. I generally use the big Sigma and also a K300/4 which is sharp wide open. The conditions often force me to use manual focus (catch in) and the results from that old 300 have been very good. I also have no thought of giving up two of my favorite wide lenses, the DA 10-17 and DA 15 Limited so APS-C is here to stay in my stable.
There still haven't been many examples posted but a quick Google search will bring up plenty of Nikon D810 shots with this lens, a camera with a similar 36 MP sensor.