At 400mm it would be crazy magnification, just above 4:1! I stick between 100 - 135mm using it mostly, beyond that you're probably going to want a tripod. I used to use one on my 100-300 too and it was above 3:1 magnification at the long end and I did get some crazy close up bug shots hand held but it's exhausting
Raynox calculator here:
http://extreme-macro.co.uk/raynox-adapter-techniques/
Your 100-400 at 100mm will give you 1.25:1 and it goes up from there as you zoom. I can tell you that AF works very good when using a Raynox, if needed, though I MF almost always with it unless I'm pulling back below 100mm for more close up than macro, just as with a dedicated macro lens AF slows up the closer in you get. It worked very nicely with AF on the Sigma 60mm I had, which gave just over 1:2 mag. I've never felt I lost any sharpness using it either, the optics are very good, unlike some of the cheapo add on filters you can get
The only issue you might experience is vignetting as the front element of the 100-400 is pretty large no? It vignettes on the 12-40 which is a shame because I had hoped to use it on there too