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I have had the Really Right Stuff replacement lens foot on my 200-400 since I first recieved it. The extent of the plate at the bottom allows for easy balance on a ball head or Wimberley with either the lens on its own, or with the 2X extender mounted, and also with either a 5D3 or a 1DX as the body. A pad might be nice for hand carry, but the rounded edges of the RRS plate are fine for me with my bare hand. If you were to use the Canon foot and add on a multi purpose plate, the lens would be a little higher than it is with the RRS dedicated foot, and I like that lower lens mounted height. The pad that is on the Canon foot to help a little with hand holding is kind of far forward, so the balance of the lens when hand held at the place where the pad is does not work out too well.
The only negative of the RRS foot I have noted is one that would be hard to fix. When you hand carry with the foot, the balance is such that you naturally would like to hold the foot a little further toward the rear of the lens, but the connecting support metal of the RRS foot is "in the way", so the lens balance is nose heavy. But to change that would require a different design with more metal further to the rear, and that would increase the weight and perhaps lower the stiffness.