Like I said before thermOdynamics, not thermaldynamics, unless you are talking about volcanos or hot springs or some crap like that
Any good suspension setup will take care of 90% of any float issues you might have at speed in any car. Period. If your car is creating so much lift at anything below 100mph that your steering feels light, I'd like to hear about it. Regarding the 350z underbody, that is nothing new, and ferrari does it a lot better. I'd like to see some comparisons between the car with and without the diffuser. If you did a test like that, I'd guarantee that the diffuser won't make a significant difference below probably 75mph. If it did, you'd see a lot of fun stuff called induced drag, translating into poor gas mileage. With lift, comes drag; you can't separate the two, unfortunately, so most aero packages are only meant to work where it counts the most - at really high speeds, where the pucker factor around a closing radius turn is enough to make you crap your pants.
The reason racing applications throw the hot air from the top of the car as opposed to bottom, is because there is always a lower pressure above the car than below it, and this causes the air to be sucked up through the hood scoop, or grille, through the radiator, and out of the top of the hood, not because the hot air causes anything to happen.