So by now we've discovered the new psyker rules: +10% for every level of PS below your psy rating that you use, -10% for every level of pushing plus increased dangers in perils and such. Why would anyone ever bother to use a PS other than just 1? What is the benefit in risking a push? A beginning psyker is too unskilled to ever reliably use his powers without maximizing his bonuses to Focus Power tests, and a veteraned psyker gains enough bonuses to pretty much garantee he will succeed every time.
So yeah, what is the benefit to pushing and why would you choose to use a PS other than 1?