I know you asked for opinions on who had started their child early but in Victoria I think you will be very very hard pressed to find someone that has a child born after April who started 4 turning 5.
The 130iq thing is only basic guideline, but it's so much more than that. My parents are both teachers (my step dad a principal for over 20 years) and he has seen 2 children in that entire time start early (ie birthdays after april) in very extraordinary cases.
It just doesn't happen.
I suggest you look into starting your child in Prep and then letting the teachers dictact whether or not they are ready to skip a grade or just be involve in gifted programs.
There is so much more to prep than just letters and numbers.