"Many other brands are mostly wood products and have very few nutrients, are too moist, etc. Add vermiculite, pearlite or sand to Super Soil to increase it's drainage and aeration." http://www.a1b2c3.com/drugs/mjgrow4e.htm
According to this, sand should be ok. Theoretically, it should improve drainage, because it does not absorb water. (thats my theory anyhoo!)