Well I had a chance to capture a crescent moon tonight between the clouds so I thought it was worth comparing the 50-200mm with 2x teleconverter (ie. 800mm telephoto reach) with hand held vs tripod with mirror up.
These are 100% crops of original jpegs (ie. you are pixel peeping at 100% here, more than you would if you printed this), and no PS processing, no sharpening.
First, at 200mm+2xTC f/3.5 (ie. 800mm eq. f/7), 1/200th sec, ISO 400 hand held with IS ON:
and, at ISO 100, 1/25thsec, f/5 (ie. 800mm eq. f/10) on tripod with mirror up (antishock = 5sec):
Well, I don’t think there is a lot of difference, so using this lens combo at 800mm hand held with IS on and 1/200th sec gives pretty impressive results and such results at this telephoto reach with autofocus hand held are unlikely to be had with any other system.
Of course, with manual focus mirror lenses you could get the reach on other cropped sensors but unless they have IS in the body, you would not get away with 1/200th sec and these lenses have less contrast and resolution compared to this lens combination.
These were taken with the Olympus E510 and look even better with a bit of sharpening such as with unsharp mask in PS.