Reasonably well is about 90+ in the subject you accelerated in. There have been some high-80's students but don't count on getting in with that mark.
I consider myself extremely lucky to have gotten into the course considering that some kids were knocked back with 92, 93 and even heard a 94 in something was knocked back where I got 89 in Mathematics + a 42/50 in MX1. It is obvious that what the Principal writes in a statement and achievement in general school work goes a fair way in the selection process.
I've been to one residential, and there is another in 2 weeks. The first was EXCELLENT and I learnt so much from the lectures and tutorials... making a lot of friends and attempting to remember a multitude of names.