Before deciding upon the 400mm DO as my flight lens, I made numerous side-by-side tests of the 300mm f?2.8 with 1.4X vs. 400mm DO. Although the resolution was similar, the 300mm +1.4X had better contrast. Of course the 300mm f/2.8 with 1.4X extender was heavier. The bare 300mmf/2.8 was both sharper and more contrasty than the 400mm DO but it was also heavier. I even made some 300mm + 2x tests which were pretty good. The basic ultra high resolution of the 300mm F2.8 is able to provide great perrformance with extenders. My 400mm Do does not like extenders. I choe the 400mm over the 300mm because of hand holding only. The 400mm is shorter and lighter than the 300mm + 1.4x and thus easier to hand hold. Even so, there a days that I wish I had purchased the 300mm f/2.8 instead of the 400mm DO. As I have said previously, the 400mm DO is a very specialized lens with two great features (size and weight) and numerous bad features (marginal resolution, poor extender performance, and low contrast). Perhaps the biggest negative is the horrible boken quality of the background. Rather than the smooth out-of-focus backgrunds of the 300mm f/2.8 and 500mm f/4, the 400mm DO produces a rough, artifacted background. Bright highligths look similar to those of the mirror lienses and line objects (like branches) appear weird. Although it is not a problem with the sky backgrounds in my flight photos, it is a real problem with tree or bush backgrounds in static bird shots. I often have to use Gaussian blur in Photoshop to smooth out the backgrounds.
Since weight is important to your application, you might want to look at the performance of the 300mm f?2.8 with both 1.4x and 2x extenders. I also own a 300mm f/4 lens. Although it is a nice light weight lens, it is not in the same category as the 300mm f?2.8. There is a huge difference between the optical quality of the two 300mm lenses. You may find that the 300mm f/2.8 along with 1.4X and 2x extenders to be a great light weight field system. It will not match the quality of the 500mm f/4. but it will be close to the 400mm Do as well as cheaper.