200-500 is good, but nothing beats trying to get closer. Now, my own experience.... Those birds in the distance are only ever going to be record shots, wish I'd realised that sooner. 600mm equivalents aren't magical and can't compare to even a decent scope, even cropped heavy. Most of those really special shots you see are close by, it's hidecraft or wildcraft that get those rather than some magical magnification of the glass. Many RSPB etc hides get you good vistas but not great photos unless you're lucky.
I will say, I sold up my various systems and now use MFT with the Panny 200 prime, though. In the U.K. we aren't blessed with great light and the 2.8 gets used more without the TC than with. That extra optical stop is worth it's weight in gold - not just for halving the ISO but for increased speed of AF.