"several inches" is a bit of an overstatement. I just measured my Halcyon plate and found the middle of the plate to be about 1 1/4" off the floor.
No, because if an STA is used it adds and additional 3/4 inch give or take. Yes, an easy two inches or even more depending on the bits and pieces and particular plate, my old Hmmerhead plate is near three inches with STA. With a roll control wing about 1.5 inches. And as far as I am concerned any additional is not needed and is detrimental. A snowball at the top of a hill has more potential energy than a snowball at the bottom of a hill, why perch the tank up on top of a center channel to begin with and then try to constrain it with an STA or roll control tabs on the wings that mostly just hang off the edge of the center channel? It makes no sense and is opposite of optimal.