Joe
IME I have actually seen higher temps on the bottom with sand in contrast to water. After the sand gets to a certain temp, you will get a radient effect. I don't think it really causes any difference in a cook, but I have heard you need to be careful with a brisket on the bottom.
Sand does have many advantages but it takes some getting use to and you have to control temp on the way up. If it gets away from you it's a problem. I use it primarily now, but sometimes I use my large Brinkman pan with water.
Good luck with the sand if you decide to try it.
Paul