Multiple scenery.cfg's : useful or not

Yes eliminates OOM errors, fly the areas you only intend to , custom configuration  with the likes of ORBX on to separate hard drives,

FSX runs smoother at better frame rates using scenery files over multiple hard drives, indexing at every change is easily solved by saving each index for that scenery cfg in to a separate folder ,ready  to load back in. I have dozens of scenery CFG,s a quick short cut from the desktop i can load in part of the world i want to fly very quickly. Also make runways for calibration purposes can also be backed up for each scenery configuration loaded. Best tweak for loading your scenery files into FSX and faster loadtimes .