You will also lose wheel up travel, but you will gain more down travel. The way you are adjusting ride height is by adjusting where the bike sits in the travel with just a rider. In order to lower the bike 1.5 inches, you are bringing the bike 1.5 inches closer to the ground. you are allowing more sag in the suspension, thereby using some up travel just to hold you. You will have more down travel in the suspension. This setup is typically better for just about all types of smooth riding. Unless you are rockcrawling the added stability in turns will be more than enough to make this scarafice in ground clearance worth it.
AC