While I appreciate the need for calibrated questions, it seems naive to expect a system to work if it depends on questions not to leak - especially if you're working with recruiters, or rejecting a lot of candidates.

What would be interesting is a system that works in spite of questions leaking. Personally I like the idea of a calibrated question pool so large that, if someone learned the answer to every question, they would have become qualified for the job. Of course you'd have to do a lot of recruiting to calibrate such a large question pool.