Well, the f2.8 of the FZ200 sounds better than it is in reality.
Compare to the SX50, the noise performence of the FZ200 is much worse.
For example, when you zoom to 600mm with the FZ200 in F2.8 iso100
and you have.. lets say a shutterspeed of 1/120s ...
you just can put the SX50 in ISO400 easily and still have the same noise performence as the FZ200.
The very good noise performence of the SX50 compare to the FZ200 is kind of
an equalizer to the constant aperture of the FZ200.
The SX50 delivers a better imagequality in general. i would say the best of all
bridge/megazooms so far.
in addition.. iso1600 is still very good usable on the SX50(especially in raw).. on the FZ200 its total crap.