Your accuracy on these problems is a great sign, as there are some really difficult questions in the diagnostic section. Did you take 75 minutes for quant & 75 for verbal? If so, that's extraordinarily fast! Just keep in mind that on the real test, you will have more time per question, but the level of the questions will adapt to your performance, so most of them won't go as quickly.
In any case, the Diagnostic is not a great tool for predicting your GMAT score. It's meant to help you focus your study on particular question types, but that is better done through an analysis of your CAT exams. For now, take it as encouragement, but make sure to take a real practice test soon, and work from there.