I don't know exactly which accounts are due to ninja mining, but it is possible to identify some of them from the early blocklogs I have found.
If you look at the current situation where the witness voting page ends at 100, there is no significant jump above/below that point, but it could just be that we need more time for that to develop.
The longer the witness list on steemit the smaller the effect it will have. If you imagine a top 30 list (when everyone has 30 votes) the easy vote will be to just vote for the top 30 and there would be a big disparity between rank 30 and rank 31.
As you increase the length of the witness list the benefit of being on the page diminishes, but having had it for long means those who are in the top 50 have had a much longer time to enjoy the benefits.