Knewton is great, i improved 110 points in 2 months.... but you are right, i am not sure how far past 650 it can take you.
i think the 50 point guarantee is a safe bet for Knewton because there is so much homework, and so many pre tests and so many lectures that if you don't improve 50 points after so much practice then something went terribly wrong. And you have to complete it all for the guarantee to be valid, as far as I understand.
So if you do all that work plus the 5 knewton practice tests you're bound to improve, even if you are at 650. But i don't think the course is specifically tailored to get people above 700...
for example, the combinatorics and permutations lesson was pretty straightforward and didn't really prepare me for the difficult 700-800 level probability questions I've seen on sample CATS.
Rates and work was also focused on 600 level concepts (in my opinion).
But it really is a great course, and a great online platform. The lecturers are fantastic. And there is so much content I think it is at least worth 50% more than what they charge.
hope that helps!