I can offer my sample size of 1. It was 12.4%. L4. "Outstanding" rating.
70% were given SI rating. The increase for SI was between 0% and 1%.
I know people with that rating who saw a much larger (10-20%) raise. I know others who got a raise in line with what you’ve said too. I don’t know the exact distribution but it’s definitely not “you get 0.5% if you’re SI”.