even i felt that knewton's test scoring methodology was not up to the mark.
So may be its just for practice purpose to practice on additional question sets. However i did not feel very positive about the kind of questions appearing in Q, if they truly reflect GMAT style.
there was one Q like..
2 3 4 5
1 -10 1... and so on 50 numbers and it was asking to add all the numbers.
I am not sure if these kind of questions appear in real GMAT.
For RC, i have seen people saying here about reading the economist
And may be you can find out if you analyze some of your stats on RC general vs. RC specific and focus on them more. Another way is to find on what kind of RCs you make most mistakes.. for me its some political issue always.. i dont get interest reading those passage.