Canon come out well ahead of Nikon, Sigma, Tamron at all focal lenght in sharpness. Nikon & Tamron performance are very similar to each others at the long end, and seems to be marginally better than the Sigma at the long end.
I would say, that on APS-C the Nikon actually comes out ahead. Contrast ratios across the frame close to 0.9 on the tele end, only sacrificing some center contrast for it compared to the Canon. At the wide end, the Canon is slightly ahead, but not by much.
On 35 mm on the other hand, the sharp drop off of the Nikon does make it more of a mixed bag, as corner sharpness is way down, compared to the Canon.
The Tamron has okay center sharpness, and is reasonably sharp across the frame for APS-C on the wide end, matching the other lenses, but high-frequency sharpness drop-off off-center at the long end is severe, and only as good as the Sigma.
Both Sigma and Tamron also are 1.3 aperture steps below the Canon/Nikon at the long end - at similar aperture values, you would expect the latter to blow the former out of the water.
As for tele converter: my gut feeling is that physically it should fit, shouldn't change the optical properties of the lens on the long end too much, but will reduce aperture values into uncomfortable territory - you'll be beyond F/8 wide open with a 1.4x, and at F/13 with a 2x. Might be good enough for the moon, but you'll also be at halving sharpness with a 2x with a 2x, which is probably not worth it.
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