I would still recommend you taking the powerprep tests.
1.Your results will outline how well you had gone through your OG
2. You can have a good feel of the type of questions (interms of difficulty) that pop up on the screen depending on how you are doing.
3. In all fairness you can expect a similar rhythm of difficulty in your actual test
Surprisingly I havent heard anyone getting 800/800 in the PP - Why is that? - especially when they have gone through those questions(most if not all). Take it - its worth it! oops. I sound like a cheap ad.