I can only speak for myself, but on custom projects, I believe that exchanges of ideas and experiences are important.
I too believe that cutting into the frame, ala the 3.9 RSR project, or a Rod Simpson type cooling system, wouldn't make me feel real secure at speed.
I would believe that the front valance design would be important and mbaran's would push more air under the car that the 930 type that I have, which, in theory, should push the air around, more than under, the car. I would think ride height would be an important variable, as well. My car is set to 25.5 inches ground to the bottoms of the fender openings.
I do know that connecting ductwork between the valance and the radiator box made a difference in fan run times and coolant temperature. The ductwork being connected made a huge (positive) difference. That being said I can only conclude that the air naturally wants to go around and under the radiator box, between the valance and the frame opening, unless forced into it.
My car is probably helped by having a the additional weight of the larger renegade full tank as well as the condenser, radiator, coolant, etc.
I suspect that the handling will change at some speed, but it won't be 140 as I'm electronically limited to 135
.
Regards,
Ken