CJ, I suggest that you go ahead and put up the drywall for 2 reasons. Since the drywall will not burn, except the paper layer on the outside, this will add a little fire protection. Do keep the drywall away from the wall by the amount recommended by the water heater and boiler manufacturer. The second reason is to enclose the fiberglass so you don't have any fibers flying around and giving you the itchies. The exposed fiberglass will also be a dust catcher and how do you clean it?