Please clarify your question. Are you telling us that you just received an 800 on an diagnostic CAT and want to know how indicative this will be of your actual GMAT score? If so, this information will be helpful:

1. Did you do the AWAs?
2. Did you complete the test in a single sitting with the appropriate 8 min breaks?
3. Which diagnostic did you take? Kaplan? MGMAT? GMATPrep? Other?
4. Where did you take the test? Was it noisy? Hot? Quiet?
5. How often did you 'guess' and get a question right by luck?

All of these variables will factor into your ultimate GMAT score. The best thing to do is take 3-4 more CATs. If your score is consistently in the 700-800 range, I think you'll do great.

"800 Score" is an online test prep company. To be honest, I don't know anyone who's used their services, so I don't have a basis for comparison. You can do a little comparison yourself, though -- compare their questions and explanations to those in OG11 and OG12 (which I recommend as the starting place for everyone).

In any case, I also don't recommend using only one test prep provider's materials, for exactly this reason; a single data point on a hypothetical graph of your performance won't give you much confidence in its validity. Every test prep company is approximating the GMAT experience, so to approximate your standing vs. the real GMAT, you should use several approximators.