This study showed an increase of up to 60 points from the most intensive test prep methods. The students in the data set would have been seniors in 1992, when 1600 was the maximum score.
http://researchnews.osu.edu/archive/satprep.htm

Probably based on the same data set, but lacking the distinctions of levels of test prep, this article claims the benefit was only 30 points, and that a great deal of families are getting fleeced due to bogus claims by test prep professionals: http://www.wsj.com/articles/SB124278685697537839

Since then the writing section was added. That section is particularly vulnerable to gaming: http://www.slate.com/articles/life/...ith_grading_instruction_and_prompts.html