200 f/2.8 + TC v 400 f/5.6?

I would like a more compact lighter 400mm for birding on my 7D. I have the Sigy 120-400 OS which I am very happy with, but want something a little more "travell friendly" for long shoots. I have heard very possitive reports on the 200 f/2.8 L, & was wondering if this would be as good or better with a 2X tc giving 400 f/5.6, than the 400 f/5.6 L white prime? How would the IQ & A/F fare against the Mighty White? Thank's.

--
lee uk.
There are old pilots, & there are bold pilots, but there are no old bold pilots.