If take a prep course and rely JUST on the prep course then you can expect to gain maybe 5 points?
However it all depends on how much effort you put in. If you can put in about 20-30 hours/week for 3 months you can expect at least a 10 point gain. It all comes down to you NOT the prep company. The prep company gives you the tools you're still the craftsman.
Taking a course doesn't mean you'll automatically get a better score. Courses provide you with resources, but it's up to you to use them well. In case you were wondering, you don't need a course to do well.