As the owner of a DA* 60-250, a DA*200 and a Tamron SP AF 300 2.8, I'm still considering a 55-300 PLM for focussing speed and for when i don't want to carry a heavier lens. The lens does have place in the world. If I had a copy, I'd do a few tests for you,
When you set up tests for a TC, you want to bak you lens up until it doesn't resolve some print in your test file, as in you can't read it anymore. The put the TC on and see if it makes it so you can read the print. Then you keep backing up until once again you can't read the print again. That establishes for you the exact range your TC will help you with.
If you can't read the print any further away with the TC, then it's worthless. However, I'm not sure I've ever tested a modern lens that bad. As I said, I wish I could help you out, I just don't own the lens.
Understanding folks saying a TC doesn't ad more detail. - PentaxForums.com
It is quite possible a lens won't give you any more resolution in some circumstances, it is unlikely it won't give you more resolution in all circumstances. The only question here is, is the 55-300 sharp enough to take advantage of a TC. After all, if the lens resolves more than the sensor can resolve, a TC will blow up unresolved detail to a size the sensor can resolve. But if the lens is so matched to the sensor that the sensor is already resolving all the resolution the lens is capable of, then the TC just enlarges what's there. A TC is only usefull when the lens out resolves the sensor.
So, I haven't seen the question answered as of yet.
For those who say it adds nothing, maybe they just didn't take the right image that shows off it's capabilities. But maybe they are pointing out the lens isn't sharp enough to benefit from a TC. It could go either way.
IN any case we can probably all agree that you'd be better off with DA*200 and the TC or the DA*300 and the TC than a 55-300 and a TC.
There does seem to be a consensus that a 150-500 or 50-500 might be as good or better than the 55-300 with the TC, but for me the whole advantage of the 55-300 is telephoto capability without a lot of weight. Something to throw in your bag in case an unexpected opportunity comes along. I'm not going to carry my 6 pound Tamron 300 looking the un-expected but I might carry a 55-300. However throwing a TC into the equation, just doesn't work for me. It's just inconsistent with what I see as my possible use for the lens.