I mention Porsche because they know more about aerodynamics than I do.
Although, my impression is that an air dam increases frontal area and produces
vena contracta along the lower edge.
A front splitter is used mainly for downforce, but it does prevent the stagnation point bleeding off into the underbody to some extent. Another Porsche example from Singer optimized for maximum downforce in off-road conditions:
If you go for an air dam follow cowmeat's (IIRC) example:
A central jet down the underbody is beneficial (apparently) and is less likely to high-center on wallowed-out gravel roads.
edit, a full width splitter no deeper than the front bumper with upturned winglets for create air curtains might be worth considering. Something like the Honda example with the tire deflectors inverted.