Long reach and light weight is a difficult combination to find. I've been through various combinations to get a balance of weight, reach, IQ, reasonable AF performance and affordability that suits me. Nothing is perfect; it's a compromise.
There are various Sigma zooms in K-mount that go to 400mm or beyond. I had the 170-500, which was one of the lightest ones at about 1.4kg. That's a fair weight to lug around, but a big difference from a DFA 150-450 or a Sigma 150-500 or 50-500, which are all around 2kg. I found it quite useable hndheld, with some practice. However the results I got from that lens on the K-30 above 300mm were not great when the subject was more than about 10m (say 30 feet) away. (Might have done better on the K-3.)
At the other end of the weight scale you have the consumer xx-300 zooms. I have the DA-L 55-300 and recently bought the DA 55-300 WR PLM. Each is a good lens, even at 300. But when you put a 1.4x teleconverter on a lens you lose one stop of light. So 300 f5.8 becomes the equivalent of roughly 420mm f8; 300 f6.3 becomes roughly 420mm f9. And that is wide open - I find the DA-L particularly needs to be stopped down at f8 or so for better sharpness at 300, so you may need to stop down further. Those numbers are really only useful in ideal conditions: you need very bright light, both because the AF will struggle to focus and because you will need fairly high shutter speeds and ISO values. And a teleconverter will magnify any limitations in the lens. I would question whether the game is worth the candle. The results may be better just from using the lens without a TC and cropping - especially if you later get a camera with a 24mp sensor.
In between are the combination of prime and TC. The sweet spot for me in K-mount, for more reach than 300, is the F/FA*300 f4.5 with a 1.4x TC. The IQ is very good even with the TC. These previous-generation primes weigh about 850-900g, so even with a TC they aren't much over 1kg, which is quite manageable handheld and not too bad to carry. You are then getting the equivalent of 420mm f6.3, and the lens is very good wide open. And used without the TC, they are so sharp that images will stand a lot of cropping (especially with the K-3). These lenses are surprising compact, although very dense.
BTW, not all teleconverters are created equal. The DA 1.4x WR one seems very good but is expensive. At the other end putting a poor quality TC on a consumer standard lens seems a sure recipe for disappointment.
One lateral suggestion. Normally I would say spend the money on a lens, but for telephoto shooting you would get good bang for buck from upgrading the body from 16mp to 24mp, especially to a K-3 which has much better AF than the K-30/K-50. A used K-3 can be had for around $US500 or so, so it would only be a $250-300 upgrade.
Last edited by Des; 02-26-2017 at 02:56 PM.