We ate in the Carnivore last March upon arrival in Nairobi. We didn't book, just turned up and there was plenty of space and tables available. You're right in going - it's absolutely fantastic, a meat lovers paradise! Be sure though, if you're staying near to Mombasa to visit the sister restauarant which is called Tamarind. If anything, this is even better than Carnivore although the focus is more towards seafood (still plenty of meat dishes though). Some people think that Tamarind consists only of the two boats they have which sail at sunset and have a lovely but restrictive menu (for obvious reasons) onboard. We ate actually at the restaurant as well, which has a superb location overlooking the Mombasa coastline.
You mention that you have a night in Nairobi before departing for your safari. If this is the case, as well as Carnivore I can also definitely recommend a visit to the Norfolk Hotel for a G&T or similar, on the Lord Delameere (spelt incorrectly I think) Terrace. This place really is wonderful and still feels very old and colonial.