This implies immediately that 75-80% haven't, and in practise that number will be higher care of the self-reporting. This substantially reduces the likely impact of 80,000 hours as a program.

Reduces it from what? There's a point at which it's more cots effective to just find new people than carrying on working to persuade existing ones. My intuition doesn't say much about whether this happy point is above or below 25%.

Good point about self-reporting potentially exaggerating the impact though.