Cyclocross magazine did some high speed photos in mud and sand and found that in sand after the initial entry at speed the extra rim depth can work against you. Actually nothing does much good in deep sand. In mud it depended more on the type of mud. Their point was that two issues are at play. Keeping mud/sand off of the top of your rim, as will happen with a typical box rim. Then you have an issue with deep rims adding friction as mud clings to the sides. They ended up suggesting that a fairly shallow V shaped rim was best for most situations and that the biggest thing was to keep up your speed. This pushes the mud/ sand away from the wheel. They failed to mention however exactly how plausible doing so is
I have used carbon rims up to 50 and under my limited capacity I think tires and skill amount to more then rim depth. I'm mostly using Edge 38 rims or Major Tom rims. For mud I prefer the Toms but some of that I think is the extra mass helps me keep up speed. That said I'm way down the food chain competition wise so take my observations with that in mind.