[Oakland Rob]

An HD-WR 55-300 is about $250, a 50-200 WR is about $150, and the TC is near $500, if you use the TC on the 50-200, you have two O-ring seals that could fail costing you near $700 + whatever body you have on it (possibly). A body + the 55-300 with only one body/lens seal, could be had for less (thinking a used K50 or similar) than the other lens combination, not including the body. You don't lose a stop of light which shooting motor-sport is pretty important.
I would hazard a bet that cropping the 200 end of the 50-200 to 280 FOV would get you about the same, if not better, results than adding the TC to it, and not cropping.

Sorry to keep beating a dead horse, but I can't see any monetary or photographic reason for this combination.
Be careful with your nomenclature too, some of the Pentax lenses and bodies are "Weather Resistant" not Weather proof. Heie's demonstration not withstanding, you need to take some care in the great outdoors with them.

Yes, you are beating a dead horse, in fact a dead off topic horse. The more polite way to do this is start your own thread about the merits of buying TCs for a particular lens.

[kh1234567890]

OK, the teleconverter arrived today. On my K-5ii the DA50-200WR autofocuses perfectly using the centre focus point and at all focal lengths. There is little, if any, difference in the rate of misses with and without the TC, even in pretty poor light. I'll take it out for a proper spin over the next few days and post some images.

[JimS_256]

I tried the Pentax 1.4 TC with my 50-200 ED WR and it focuses normally (no worse than the lens without the TC), even in poor light.

[kh1234567890]

Well, I got the teleconverter, played with it for a day or two - and returned it. On both the DA50-200 and the DA55-300 there was no advantage to be gained as opposed to simple cropping and resizing. I tried it on some of my other, shorter, lenses with similarly disappointing results. Perhaps the worst was the DA70 Ltd, which became unusably soft with the TC on it.