I do use the Gitzo 3541L monopod with a Acratech ballhead if I need to. How do you think the AF on the 300mm IS Ver with the 1.4xTC compared to the bare lens itself? I have laid my hands on the 400 IS ver I a few times and I have to say that's a heavy chunk of lens, it will be impossible for me to handhold.
I honestly don't notice that much of a difference in AF speed, I guess that's because 50% of very-very fast is still very fast. There is a slight hit in IQ, but that lens is so sharp, that it's still very good and I hate to compromise and I'm pretty picky.
I actually went to the Camera Shop to buy the 400mm f/2.8L IS and the pro sports photographer that worked there, talked me into the 300mm f/2.8L IS. I'm using it with the 1D MKIV. The 400mm is the ultimate field sports lens, however it's pretty long for indoor sports, and it's a real beast. The 300mm with or without the 1.4X, is the good all-arounder for baseball, basketball, volleyball, hockey, motor-sports, cycling, Air Shows, and I even use it for Surfing with the 1.4X and some mild cropping. I also use it for Birds in flight and it's not that bad to travel with it.
If you are going to be a professional sports photographer and you asked me what's the best lens for Football, Soccer, and Lacrosse, then I would say the 400mm. If you asked me what's the better all-around lens for many different sports, then I would say the 300mm.
It's great that you have a monopod, but ballheads and monopods are like drinking and driving, they just don't mix. You can just screw the lens onto the monopod. If you would like to use it for BIF, then an RRS Monopod Head with quick release is great!