I understand the Dvorak keyboard well enough, and just how it produces that greater efficiency. It is almost entirely about the mechanics of touch typing; more efficient key placement to distribute the work between hands, and the most common keys closest to the home rows and stronger fingers.
I think you underestimate how much a cramped, non-standard, touch keyboard is going to impact that efficiency. There have been a few, very few, people who have reported being able to touch-type with the iPad's touch keyboard with any real efficiency. Dvorak is all about making touch typing efficient.
So I maintain, that except for those few who are comfortable with holding their hands above a keyboard with 'no' contact, and have the spacial/motor skills to still be accurate that way; the primary benefit would be finding the keys where you are trained to expect them. I don't deny that there should be some improvements. Just having the more heavily used keys a bit closer together will help some; but hen pecking is hen pecking. A 38% efficiency improvement proven on a physical keyboard does not necessarily translate to the the physical and layout compromises inherent to the iPad's hardware.
It is just my opinion of course, and I have no studies to back this up. If you do, specific to the type of screen/keyboard involved, I will be happy to change my opinion.
Hold in mind I have no objection to the idea of Apple including a Dvorak keyboard. I even think they should. I believe the only reason they did not was because they were concentrating on supporting regional keyboard styles, not professional ones: They will probably get around to it eventually. You could probably help hurry that on it's way by giving them some feedback, if you haven't already.
Apple - iPad - Feedback