African Safari Lens Options

I am planning to take my 18 yr old son on a safari early next year for his graduation (code for I want to go back to Africa but need an excuse). I have been to Tanzania (2009) and Botswana (2005) in the past and am planning on visiting Kenya (Massai Mara) and Zambia (South Luangwa). I currently have a 7D and a 100-400L lens which worked well last time but will need a second "long" lens for my son's camera (40D). Seems like I have the following options:

1. Hope the new 100-400L is released by then.

2. Sigma 50-500 (Bigma) with OS - better range but maybe optically not as good as canon gear.

3. Sigma 120-300 2.8 - faster lens for early morning and evening shots. Could be used with extender.
4. Canon 400mm 5.6L (no IS may be a problem)
5. Canon 300mm 4.0 IS with 1.4x extender

I would love to get the new 200-400 4.0 with build in extender but expecting that to be out of my price range, as is the 300mm 2.8 IS or 400mm 2.8 IS.

Renting is an option, but wouldn't mind purchasing one for future wildlife and outdoor sport shooting.

Any advice on the above options would be appreciated...