I have actually designed and built a small DC (less than 5K sqrft) using this method. Cost was the major reason a raised floor was not put in place. Plus the fact that most areas require ramps for their raised floor which eats into the DC real estate.
We had this done and we were able to sufficiently cool the DC and we provided the proper ducting to direct the supply and return air streams to help "optimize" the plan. I stay in frequent touch with the DC and they are running just fine and we are looking at additional capacity for growth.
Dave
cinkdave@gmail.com