It's similar in design to how intrepid builds their transom.
I'm not sure what you mean by this question . . "I wonder if the water separates off the transom enough to reduce the drag with such a short extension?"
you see any high performance racing boats or bass boats with 30" setbacks? nope! Most jack plates are 10" maximum. The bracket is 4" off the water when you are running . . . so it's a non issue. The reason the current armstrong, hermco, stainless is deigned the way it is now is so you can convert/retrofit an i/o and get full tilt of the outboard with a nice swim platform. None of the popular brackets are designed with performance in mind.