Would this help with underhood temps?
 

Yes

haha

NO. No it won't. I'd bet that underhood temps would be lower without the plate. However, I'd also bet that coolant and intake temps would be lower with the plate.

I'd rock the plate.

B

If it is installed properly it will direct air flow thru the rad instead of over the rad.

Add a dust pan scoop under the car along with the rad cover and more more air will flow thru the rad and ic.

Then add an extracter hood to complete the picture.