Here is one for folks who might like to put together long reach combo's with the 7D2. I shot for an hour or so this morning at Conowingo Dam handheld with the 7D2, 400 5.6L and 1.4x ii tele yielding an effective focal length of approximately 900mm. Sadly, the results were not very good. The 7D2 seems to focus fine at f8. It even tracked reasonably well. You can't use the Zone AF modes but you can use assisted center point and it had no trouble holding focus.
I think the problem probably is the 400 5.6L. I think the 7D2's resolution is about all it can take. It's an old design and I suppose modern ultra-high res sensors may have caught up to the pocket rocket. The 1.4x just puts it over the edge. 900mm is a lot to ask from such a small object lens. Even at 1/3200s the photos are mushy at 100%. I generally like 1/4x-focal-length for BIF's so maybe 1/3600 or 1/4000 would be better. But, stop down to f11 to help sharpness and now you are maxing the ISO.
So, maybe on a tripod for something that moves slower than Bald Eagles you may be able to get something decent but I wouldn't count on it. YMMV. A new body always seems to trigger another Lens upgrade cycle.
Anyway, here is one that is just about as good as I could do. It's not cropped at all. Cropping these was a non-starter. It works fine as a small image but even at page size it's pretty mushy. I think these would likely have been better shot without the 1.4x and cropped but I'm not sure. That is always tough to call.
I suppose if I was just trying to capture a rare animal or some unusual behavior it would be fine as a photo record if that is what you are into. - Brian