Single digits for most of the night for sure. In May you will reach close to zero depending on where you are on the course about an hour before sunrise. It's freezing going down on other side of Rooiberg for example.
I have not done one where you start at 15:00. All mine was a 18:00 start time which kinda helped cause it started getting cold already so the way you dress at the start is probably how you go through the night. I think in April ones I took a rolled up jacket with in my back pocket but had a long sleeve undervest on, then put the jacket on at CP1. Again the start time here was diff to what you deal with now.
The climbs heat you up, the downhills cool you down so it's a tricky balance of not building up a massive sweat on the up that will freeze you to death on the downs.
With the 15:00 start you might have less heat to deal with next day as well, but that also depends on your riding time. I didn't record more than 20C at 12:00 but I do remember 2017 it being very hot. Dump what you can in Calitzdorp box.
PS: I didn't use leg warmers.