We always play level 3. I'd say that a 10 point score increase sounds about right. Even higher scores are possible.
I tend to agree with Mattias. "FotM" injects extra timber into the game, along with more points via horses and a ton of new Majors. The highest score I've ever managed was a 2-P Level3 "FotM" session with "Through the Seasons" added. It all depends on how many players there are and how outright competitive they are.