I bought a canon 300mm f2.8 is and have the sigma 300mm f2.8 lens in pentax mount. i thought i would give my opinion here. First the canon 300mm lens and 7-d is an excellent lens, but the K-5 in my opinion blows away the 7-d. In the jungle the 7-d is so noisy at high iso it is almost useless. Where the k-5 is just blowing and going with no problem.
Second the lenses. This really thru me. The sigma 300mm f2.8 prime is just as sharp as the canon 300mm prime. The difference is not enough to talk about. The sigma prime is excellent. as is the canon.
My partner here is using the pentax k-5 and i the 7-d. These pics were shot at the same time at the same distance.
I also own the sigma 500mm f4.5 lens and the canon 500mm f4l is lens. The sigma is a tad sharper than the 300mm lens. So i expect it to be in the same quality of pic as the canon. when i have the opportunity i will check.
The canon mtf is total nonsence. But i think the sigma is right on spec. kinda strange i think. But in my opinion the sigma with pentax is almost as sharp as the canon. I think the canon edges it out in heavy crops. But with the size of the Canon vs. size of the Pentax and sigma. In the jungle i choose the pentax. Very easy to handle.
I thought I would tell you I replaced the older Canon 300mm f2.8 IS with the new 300mm IS 11. And it is an unreal lens for me. Much better than the older 300 IS for me anyway. So Canon slammed that one out of the park. It really is much better than Sigma for sure. IS is amazing. So Canon wines for sure now. Anyway just a bit of trivial news.