Hi Alex! Welcome to GMAT Club.
GMAT prep is fairly accurate and usually you will score within 50 points. If you have taken other prep tests, take a look at the GMAT Score Estimator/Calculator
Really depends on how you handle stress - some people perform better and some don't.
I scored exactly 600 on the 2nd GMATprep test that I downloaded from the mba(dot)com
What's my chance on getting the same score on the real GMAT (providing I tried to simulate test conditions as close as possible). Will it differ too much?
Founder of GMAT Club
Just starting out with GMAT? Start here... | Want to know your GMAT Score? Try GMAT Score Estimator
Need GMAT Book Recommendations? Best GMAT Books
Co-author of the GMAT Club tests
Have a blog? Feature it on GMAT Club!
Get the best GMAT Prep Resources with GMAT Club Premium Membership