I rarely use anything beyond 300 mm, and for the price and image quality the Tamron 300 mm/2.8 is hard to beat. In Africa you can get reasonably close to most common large mammals. IŽve used larger lenses mainly for small birds.
IŽve never been on a tour, always have driven my own car, on my own schedule. Tanzania and Kenia have the worst driving youŽll ever see, so maybe hiring a driver is not such a bad idea. And be careful when you walk dowtown Nairobi. ThereŽs a lot of street crime. Good luck.