It also depends on the design of your MDL terrain. I've also found out, through a fairly recent experiment, if the mesh is regular, such as a grid you'll get a faster frame rate. If irregular, I've noticed a 15% drop in the frame rate (I don't recall the actual result I got; based on this being the only thing seen (and duplicated many times using 1024 polygons or something like that). MDL terrains also allow you to work around corners for odd-shaped buildings and objects as you can just cut out the unneccessary part.