It works fine on the 100-400. I use mine all the time shooting motorsport, have a look at the Donnington 2008 shots in the link below and judge for yourself. Even in the pouring rain its fine.
I use the Kenko PRO 1.4x, it is better quality, cheaper and more usable then Canons own TC's, which are limited to a certain range of lenses.
Kenko do at least two different grades of 1.4x. The cheaper version has no trouble with chatting to the lens because it has fewer pins and I imagine thus chats less! The problem is that the quality is not as good. The better, PRO version does require three pins be taped over - its not a big deal and seems to have no downside that I've noticed.
In theory the lens might hunt more, but to be honest I've never found it an issue on my 30D or Eos-3. AI Servo is all I ever use trackside, it does need me to concentrate on keeping the focus point nailed when a bike is doing anything from 100 to 200mph, but thats just technique.