So please explain how that is "outperforming" it?
Simply, the detail or resolution from the SX50 at 1200mm is better than the 5DIII +100-400mm at 400mm. If you take a crop from the 5DIII combo to give the same view as the SX50, the SX50 image is better. This is useful if you are taking a bird photo which requires heavy cropping from the 5DIII and 6x less from the SX50.
As I had in the first post, I am not implying the SX50 is better than the 5DIII but the extra reach at 1200mm beats out the 100-400mm at 400mm. That is quite remarkable and unexpected to me, and it means I can travel light. Normally, I would use the Canon 5DIII with a 600mm prime, and only use the 100-400mm when I had to travel with limited luggage. But, for work trips I'll just take the SX50 for from now on knowing I can get some spectacular shots.
So you're taking a crop where only 1 pixel out of every 9 on the 5D Mark III is thrown out, and you're saying that a point and shoot is better.
This would require your 5D Mark III to have 10 TIMES the starting resolution to get a better result.
No wonder you came to the conclusion you did. Now try the 5D Mark III with a 2x teleconverter and a 400mm lens vs the SX50 @ 800mm.
Please read everything that has been written here. I routinely use a 5DIII with 600mm of L lens attached for bird photography and am happy to cart this gear around because it gets superlative results. When I travel light by plane or have a few minutes to spare on a business trip, I usually take the 5DIII with a 100-400mm for bird photography, which is still quite heavy and not in the same IQ league as the prime lens. I have found that for bird photography I can use the SX50 and sometimes get better results with the 100-400 set up.
Regarding your suggestion of putting a 2xTC on a 400, I have tried a 1.4x on the 100-400mm L and the 5DIII. Aside from the autofocus being horrible, the IQ is degraded so much that there is no advantage in using the lens without TC and rezzing up 1.4x. A 2xTC would be even worse.
Regarding the comment by RGF about not being useful to most photographers, CanonRumors represents only a tiny fraction of photographers - those of us who are primarily interested in high end gear and the highest IQ. There are 100s of millions of photographers who want tolerably good kit that they can afford and easily use to get acceptable results. Cameras like the SX50 have huge popular appeal and can be useful to us as well.
5D IV, 5DS R, 7D II, EOS-M, Powershot G3 X, 400mm DO II, 300/2.8 II, 1.4xTC III, 2xTC III, EF 1.8 STM, EF 70-200/4 IS, EF 24-105, 15-85, 100-400 II, Sigma 10-20, Sigma 150-600mm C, EOS-M 18-55, f/2 22, 11-22