I think September is an excellent time to go. I would say that is especially the case if you include a buffalo in your hunt (something I would highly recommend). HHK has a great buff/sable/plains game package (12 days). And their buff/plains game deal (10 days) is nice, too.
Getting up each morning, heading towards water, and following the buff tracks either from the water itself, or from some other spot where you pick them up after they have had their drink, is awesome. Hunting near water sources is also helpful with plains game, though probably not as critical or predictable as with buffalo.
The thing you just want to be careful about is that they put you in sections of Lemco that have plenty of what you're looking for. As I mentioned about what happened with me, the section where I was hunting sable had very few. It was frustrating to see sable in other spots and I was unable to hunt them because the quota there was gone. Mention this to Graham. He'll know what I'm talking about. My HHK PH, Howard Hunter (who may be able to hunt in 2005 after recovering from being shot by a client last July), assured me that the next time I hunt with them, he would make certain that everything I wanted would be available. This is where a previous relationship with a good PH can start paying dividends.
That said, I hope to get my sable when hunting in Tanzania for 21 days next July.