I just switched back from a billet grille, and it makes a real difference (~20 degrees in hot weather) in engine coolant temps. I'll probably put it back on in winter.
It didn't seem to impact transmission cooling as much (though obviously the oil to water cooler will work better), in fact I went to it in order to make room for a small electric fan to run at low speeds, I finally just hacked up the OEM grille to do the same.
I've been using one of these:
http://www.amazon.com/gp/product/B00390 ... detailpage
on the transmission cooler, and it does make a real difference at low speeds, but it's not even remotely in the same class as the main, engine driven fan.
I also pulled the hood off in desperation while towing a heavy equipment trailer in stop and go traffic last summer. In my experience, it does nothing to keep things from heating up when the engine is doing work, but it does help things cool down again much more quickly when it is not.