Dave, congratulations on being over with the test. Secondly, as much as I understand how you feel, don't discard the possibility of you actually scoring low. Remember how the CAT works, if you answer a question right, you get a tougher one, if not an easier one, etc.
So, for eg., you may have answered wrongly 2 or 3 questions out of the first five and then 1 out 3 or 4 right for the rest of the exam and score a 20. The problem is you would have been answering easy questions right and not getting much points for doing so.
Additionally, don't rely too much on being a native speaker. In GMAT you'd be running out of time constantly if you have not developed a specific approach to the test.
Furthermore, eventhough OG is a good predictor of actual performance, it is a good predictor of post-preparation performance. Eg: you take OG sometime within your preparation and score 700. Then you prepare for an extra month, really get in the mood, and score around 700 in the actual exam. But scoring 650 - 700 in OG and sitting for GMAT the following day may yield anything from 500 to 650.
Hope it helps. L.