John,
I have the 400 f2.8L IS, and will tell you that it is heavy. I use it mostly for shooting action sports with the 1-series bodies, and use a monopod for that. (I mounted an RRS plate to the tripod handle, and use a QR plate atop the monopod to easily separate them for carry.) I have an older ArcaSwiss B1 ballhead on a very sturdy tripod, and would not feel uncomfortable using that, but you are approaching the limits with the BH-40, I think. (RRS does tend to be a bit more conservative, so you may have a bit of room to spare, but should the ball loosen and the camera/lens flop over, you could put some serious stress on the ballhead.)
Most folks using the 400 for action, use it on a sturdy monopod for both ease of carrying and positioning. RRS has a new monopod head, MH-01, that I have seriously been considering to permit great vertical plane motion that is not easy on a monopod without lots of bending back and forth.
On a tripod mount, many have gone to using the Wimberley Head to provide greater flexibility and panning action for things like birds in flight. One can use the Wimberley Sidekick also, which mounts to a sturdy ballhead, but I do not think the BH-40 would hold it securely enough.
Anyway, congrats on the new glass. It is stellar for sure, but it will give you a workout if you have to carry it around a lot. When I shoot polo and stuff, I have the 400 + 1-series body on the monopod, a 70-200 + another 1-series body around my neck, and a third 1-series body with a 24-70 in holster or hanging from a shoulder for closer in shooting. The 400 is an armful at times, but nothing delivers anything close to what it can do, especially wide open, if needed. Just keep your shutter speed up, if you can
LJ