Do the cooling plates that I see for the Z actually make a difference in engine temps or is it all for flash?

 

???? i have one

 

I read on a review on a magazine where they tested an S14 240SX with and without a cooling plate. At low speeds it did nothing, but anything above 3rd with sustained high revs and it lowered outlet temps on the radiator by up to 10F. So i'm not sure about our exact car but it certainly worked for the 240.
You have to make sure that it is a plate that seals the entire top end of the rad, not just a little piece that covers the bonnet latch. Those are pretty useless.

 

^^ Which brand seals it off fully?

 

I'm not sure but i think the autotechnic one i've seen roaming around ebay does.