If 560mm was enough for me, I'd be interested in this lens. As it is, I'm usually using at least the 1.4xIII on my 600 II for 840mm f/5.6, and sometimes I go to 1200mm f/8 with the 2xIII.

I'm not sure f/4 (much less f/5.6) is optimal for sports, even with the high ISO performance of current FF bodies, in the 200-400mm range one often wants the additional subject isolation that an f/2.8 lens delivers.

I can see the 200-400/1.4x as the ideal single tele lens solution on a safari, for example. But then, it's also worth considering that a package consisting of a 1D X, 300/2.8 II and 2xIII is not too far off in cost from this new lens...