Having been through the system and been successful, I'd say it's more like 25% are successful, if that. And that was nearly 2 years ago when they were desperate for cadets!
I had never flown a 738 before the assessment day and paid for some time in a full motion 738 sim, not so much to get a head start, but to make sure I felt comfortable with the layout and feel of the sim. I am convinced that this was worthwhile and the proof is in the pudding: I had a good assessment day and passed. My sim buddy had a mare and got a rejection email within days.
I also had a good-humoured and friendly interview. Remember, whatever you may have heard about FR, they're not just looking for people who can be trained, they want people who are socially adept. Be friendly, cheerful, smile, but know your technical stuff.
PS: 10 minutes in the sim is normal, but yes, the TRE will know within seconds whether you have the aptitude to fly a jet. He doesn't need 30 minutes of hold entries or raw data ILS approaches to form a view!!
PPS: I gave the assessment day a substantial investment of time. money and effort. 2 or 3 years ago there were few other options, if any, and my gamble paid off.
These days there are other airlines recruiting...