Here's an older article, but for me the science checks out:
http://www.wetwebmedia.com/deepsandbeds.htm
Good article.
I see now why you reccomend against having a sandbed at around the 3" mark. Still, my original point stands where as long as you have an adequate micro fauna and CUC population, and you're able to get adequate turnover of your sand bed, you really won't see any negative effects to having a sand bed at about that depth