I think your comment needs some more discussion before we move on.
What stands out to me:
1. I agree that the positive intent of the spoiler can often be negated by other features, sometimes not always first apparent.
2. In the OP's case, any air the pictured spoiler redirects, has to restrict an certain amount of under car air flow, that is a plus.
3. Some of the redirected air, but not all of this redirected air will find a path into engine compartment.
4. There are two main reasons for that, the pressure in front will be higher, and the pressure behind will now be lower, because of the under car airflow has been restricted by this spoiler.
5. It is not difficult to restrict this now higher pressure area entering engine compartment in the front, by adding simple well designed blockage, but still retaining adequate airflow for cooling.
6. The high pressure air that now has been restricted thru the radiator, has three other pathways available, back under the car, least likely, around the sides, and over the hood.
7. In the last example above, and IMO, often the most likely path, the high pressure in front will exert pressure on the underside ( and topside) of any non vertical surfaces, like the bumper ( which in the OP's car really is not a large horizontal area, as bumpers go)
8. However, this high pressure area will also exert an almost equal downforce in the same bumper from the top, showing no real change in DF from this bumper issue.
9. The real gain in DF will be from any additional flow over the hood as noted, and any reduction also found in under hood pressure.
Other then that we agree.
This is my solution in a similar car pic #73, note in this pic the upper grille is completely block with lexan, the brake ducks have not been enlarged, and the spoiler has separate end vanes, and with short rubber extension for streetability as I noted earlier.
https://board.moparts.org/ubbthreads/ubbt...tml#Post1553895EDir #2 I made to clarify my response, Craig's issue with underbumper air build negating DF certainly has merit, and is as he mentions, difficult to overcome with our older bumper set-ups
