Hi SW,
As a birder, I can tell you that you will probably always be presented with choices between new bodies and longer lenses. This will be a long post. . .
Here's how I see it, and by no means can my opinion in these matters be considered mainstream.
Most commonly, photographers will tell you it's not the gear, it's the photographer, but in this particular situation, there are ways that both lens reach and a new body might improve your photographic experience.
The differences between the K20 and K-5 are significant. Focusing speed is very noticeably faster and more positive (fewer or no micro adjustments before lock) between these models. In addition, the AF sensor is considerably more sensitive, so I'd expect at least a stop difference in the light levels where the focus will lock with a given lens. Also, the faster frame rate, while handy in continuous shooting, also gives you shorter VF blackout periods, and makes it easier to track moving subjects and pick the poses that you want to capture. The shutter is vastly quieter, and if you've ever had animals startled by the shutter sound, this will be considerably less likely.
High ISO performance is a revelation for me. I see 1/90, f4 (wide open) ISO 400 in the shot you posted. I'm just getting started shooting the K-5 in the field, but right now, I'd not hesitate to shoot this at ISO 3200, which would give you 3 full stops of leeway, so you'd have some options to stop down and still shoot at a considerably faster shutter speed (this would probably give you a similar noise profile to what you have here). A good NR program is going to be necessary to get the most out of the very high ISO features though. . .
The Bigma is a good lens, but it's big
. . . About 2 lbs heavier than your DA* 300.
I like to shoot longer, but have physical limitations to what I can carry out in the field and insure that it all gets back to the car at the end of the day. I've limited myself to @ 6 lbs in a give lens, and that means 300/2.8. I go longer with TCs.
With your DA*, there are no real SDM TCs, so that's a problem. But the Pentax F 1.7x Auto Focusing Adapter has worked well with every lens I've ever tried it on. I'm sure that the DA* will play well with it, and if you really want to extend your reach, and will probably continue shooting wildlife, then it's a pretty suer bet that this piece will save you money in your pursuit of more reach.
There are some caveats. The AFA was not really designed as a TC. it was designed to transition Pentaxians from MF to AF with a minimum of cost. The 1.7x magnification is a necessary function of the added length of the adapter to have enough room for the movement of the focusing group. The AFA allows full range focusing with a 50mm lens that is focused to infinity attached, but with tele lenses, the range of distances where it can focus is more limited, and the longer the tele, the more limited the range. This requires a user to manually prefocus the lens to get it into the range that the AFA can handle, then the AFA takes over and achieves final focus. This sounds more involved than it is, and once one gets used to it, it's second nature but some people hate it.
Personally, I look at this "fault" as an advantage. I see it as an easily adjusted focus limiter. AF with the AFA is very quick once your within range since the focusing group is small and light, and has a very short throw. Hate those lock to lock focus hunts when you just wanted to make a slight focus adjustment between shots? Not a problem with the AFA -- lock to lock is maybe 1/10 of a second or less, and reacquiring a new lock is usually about the same since the subject never gets so OOF that the AF system can't find a lock.
There is light loss -- there has to be -- but your f4 300mm will turn into a 510mm f 6.8, or 1/3 of a stop slower than the published max aperture for the Bigma, but at the cost of a few ounces, not pounds. The Bigma also needs to extend to reach its 500mm long side -- you will most likely see this lens pictured at its 50mm setting. This can be a problem if you choose to use a tripod and gimbal, since the gimbal relies on balance, and if you want to use the zoom feature of the lens, the balance will change. With the DA* and AFA, you have no zoom and internal focusing, so the balance doesn't change.
If you want to go longer than 510mm, then you have some options. 300mm f2.8s give you the option of adding another TC because the extra stop of max aperture can absorb the 1 stop that a 1.4x TC would take away, so with a 1.4x+1.7x, you'd get 714mm f6,8. You also have the option of buying an MF 300/2.8 since the AFA will do the focusing for you, and that means you can save maybe $1000 over an AF 300/2.8. . . There are other things, but that's for future consideration.
Bottom line. The K-5 will give you a significantly more capable and more refined body to shoot wildlife with. The AFA, if you can get one for a reasonable price and like using it can give you a lot of versatility in future lens choices and save major money -- ultra tele lenses are expensive. I've been told from the start that I'd be better off shooting Canon, Nikon, or even Olympus for birding. I've stuck to Pentax, and I have been happy with the choice from the DS through the K-7, and am much happier since getting a K-5.
Just my 2¢ . . . or probably more like $20 since I'm so wordy. . .
BTW, Neat Otter shot!
Scott