Something I had never read before is that the Image Stabilisation will decrease the speed of the camera, both in frames-per-second and autofocus. Untill now I only could get 7 fps out of the D1m4, which is way slower than the 10 fps in the folder. But with the 400mm attached the speed improved significantly! And indeed, when IS is disabled on the 300mm the camera also gets 2-3 more frames per second. The same holds for the speed of focussing. The IS image stabilisation drains so much power from the camera that it loses performance.
The difference in image quality was also surprising. Note that I am always looking for sharpness on actual pixel level, I prefer cropping instead of investing in 500/600mm and having to carry the weight. The image below shows the actual-pixel output of the 300 and 400mm on ISO640, rather a big difference (some additional noise may be added by blogspot-compression).
|left: 400mm f5.6 (on aperture 7.1) right: 300mm f4.0 IS (on aperture 5.6)|
|the 300mm is a winner on short focus-range with only 1.5 meters|