I did have the game running at 60fps with vsync enabled with both the modernizer, and without.
I don't believe it has something to do with the fps however, simply because without the modernizer it was a lot easier if it had to do with the a.i. difficulty being "tied in" with the fps then without the mod it would have been equally as difficult as with it installed.
It's a good thing to know for Halo though, I've recently been wanting to replay the games on legendary as well, so thanks for the advice.