I just did my check, and there are 814 members that have above 250 posts.the list drops off sharply too, so there are only 2660 members that have a significant amount of posts (taken to be 50, in this case)
link
Doing a bit of math, that means we have:
0.3% of all users are users that have more than 1000 posts.
2.6% of all users are users that have more than 250 posts.
8.2% of all users are users that have more than 50 posts.
However, if you look at participation rates compared to those before the board was made into a participation based board, they are a marked improvement.
Hope the solid numbers help a bit.