The EC-20 is a nice piece of glass. Be prepared for a dim viewfinder and the need for alot of light to keep shutter speeds up for a very long lens.
I don't use it with the 50-200 (I have that other option
), but it gets alot of use with the 35 macro (True 2:1, 35mm equiv 4:1
) which is the same brightness as the long end of the 50-200.
My favorite (for macro) is the 50 f2 macro with the EC-20, and it is a very easy to use, easy to light (with flash) and offers outstanding IQ, even well stopped down.
It also gets alot of use with the 300mm, and I'm certainly pleased with that combo.
I'm guessing that the EC-20/50-200 might do well for closeups also. It will retain every milimeter of working distance and double the magnification.
You will enjoy the reach. 400mm on a 2x factor is getting out there. Have fun with it when your lens gets back.