I've no experience so I can only go on what I've read.
From what I read the
AF is slowed by the
IS, but with the shutter speeds you're likely to be using you're unlikely to use
IS.
For your purposes the most flexible will be the 100-400 rather than taking the t/c on and off, but the
AF on the 300 alone will be faster than on the 100-400. Now, how many fractions of a second difference probably isn't a readily available number.
You may have to hire them and try them out before committing to one or the other.