If the mudflap is less than 5" at ride height then it can throw up rocks, etc. A hard shield can reflect rocks forward putting paint and rear radiators at risk. The best protector is a Protect A Tow type to keep everything under the car.
If I had a shield I'd cover it with say a shower rubber mat from HD, etc to reduce deflections forward.
I'm very doubtful that a shield that far back would have any significant effect on air flow. Now if you drive with the shield stowed on the rig then it seems likely to have a negative effect.