My 2c: The three questions meant to sort the wheat from the chaff don't go nearly far enough. Have a designer write them or at least review them. As they are now there is a million and a half Pinterest users that can answer with a resounding Yes! to all three of them, and only a handful of them will have the grit you are looking for.
There is this dynamic that you need to pinpoint that "A-star" (as you call them) people too want to move upwards in their career, so they want to apply to jobs (if ever) where even they seem just the right amount of under-qualified.
My suggestion for nailing this dynamic would be to up the bonus requirements by adding stuff like this:
* Designed an app spending significant time in the US top 20
* Introduced an interface metaphor on iOS that got copied by Instagram or Evernote
* Worked with graphic design close to the core iOs team at Apple
If you see what I mean? The thought you want to put in the mind of your dream candidate is "Oh man, I wish I had what it takes to apply to this team".
And the mechanic you then apply to get to talk to the right people is to look around Dribbble, Forrst, Lovedsgn and Deviantart and point designers to the ad and say "We think you might have what it takes to be part of our team. Do you want to have a conversation about it?"
Sell your team by setting the bar for entry.