Yeah, pull the trim off; reminiscent of Mike Kelly's fender vents. He has a relatively large hole in the fender, but then he put a cowl style vent on there and it covers over the huge hole leaving only much smaller, maybe 1x6 inch hole at the end of the vent. It will work, and so will yours, but if you think you still have pressure under the hood, utilizing more of the hole in the hood should help that pressure escape.
Your comment sounded like you couldn't find the test data: http://forums.hybrid...nnel-test-data/
You can see by the lift in the front end on the aero testing that there was a lot of pressure under the hood, but they didn't use any rad ducts on any of the cars, which should have a huge influence. I think a few cars had a plate from the valence to the core support, but nothing to prevent air from going around or over the core support.
In terms of where to exit rad air, there are problems with any solution. If you go out the sides, then your downforce gets disrupted by cars driving next to you. This is a key to NASCAR racing. If you have it come out the top of the hood, then you're feeding hot air to the wing if you have one. No perfect solution here. You really don't want it exiting out the bottom.
Yes, low pressure under the hood, or even lower than the pressure in the rad duct will draw air through the rad. Still needs to go somewhere though, and a dedicated exhaust that takes all the air coming through and sends it out the top or the sides will help to pull air through the rad but also keeps that air from filling up the area under the hood. Add in some louvered vents to pull more air out and you should really be able to get some low pressure under there.