I have taken a number good Southern impala,and I can say the biggest I have taken overall was on Louis on his personal farm, my wife "swat"ed a monster (25 1/2 in) a monster by southern impala standards in 2012.
I took a nice on there too, but my was just a little shy of 25 inches, but he was wide!
We have seen impala on that place that may have neared 27 inches, there is one on the vid that they have a fleeting shot of, he is near 27 inches...monster.
Louis is a good dude,and the experience he give you is a good one, I have hunted a lot in Southern Africa and can tell you, it's a good one. So many now days you blast from the truck, or hunt in a tiny camp. I love wild Africa, but unfortunately, it is drying up. The beautiful Botswana, I saw in 07 has gone away, and most other destinations like the Caprivi and Zim (Save,matetsi) are either shot out or very expensive. It's a good option. Especially for the first timer!