Now we see through a glass, darkly...
- Apr 5, 2016
The diffraction limited aperture of a 32 Mpx APS-C sensor is f/5.2. An f/7.1 is basically reducing it to the resolution of about a 20 Mpx APS-C sensor. I had a 90D, a great little camera with a 32 Mpx sensor, but it needed my 400mm f/4 to take advantage of it. On the 5DSR, which is about equivalent to a 20 Mpx APS-C sensor, the 400mm f/4 was hardly better than the 100-400mm II at f/5.6. But, on the 90D, it was much better. So, you would have to pay a lot for a lens and carry a lot more weight to make 32 Mpx APS-C more worthwhile in practice than a 20 Mpx sensor.
On the other hand, for the same angle of view as an uncropped FF camera that a 400mm f/4 gives you, you can get away with 250mm f/4, which isn't that far away from a lighter, more flexible, and much more affordable 70-200mm f/2.8 compared to a 400/4.
Now, put a 1.4X on the 200/2.8 and you have a 280/4, which is slightly more reach on an APS-C sensor than a 400/4 on an uncropped FF if both sensors have the same MP count. In my experience, you don't give away much in terms of IQ that would show up in the typical use cases (sports, birding) using one of Canon's latest EF 70-200/2.8 lenses with their latest EF 1.4X extenders. In fact, the extender tends to counteract the geometric distortion of the 70-200mm zooms when used at the longer focal length end, which is where one would typically use a zoom lens with an extender.
At 280mm and f/4, you're still below the DLA for a 32MP APS-C sensor.