A: Depends on the vehicle.
The good news is it is relatively easy to mock something up on your vehicle (CAD - cardboard aided design) and test it. (Stopwatch results of multiple coastdown runs with the dam on / off / on immediately following one another in identical conditions will reveal your answer.)
For example, see:
Airdam - suprise result! - Fuel Economy, Hypermiling, EcoModding News and Forum - EcoModder.com
No splitter, and not quite a ground-scraper, but you get the idea. (You also see that it apparently did not help his second vehicle, thus the "it depends" statement.)