First attempt.

Yes, if you are far from your target score, it is highly unlikely that you will achieve it in 7 days.

Whether you should re-schedule depends on how you feel about the financial aspect of a retake. There has been a great recent development in the GMAT world - when you cancel your score, the schools do not get to see the canceled attempt. So this means that if you do take the test and do not get a satisfactory score, you can cancel it and no one will be the wiser (except you). You might wonder why you would take the test at all if you are expecting an unsatisfactory score? It's a brilliant practice run for the actual test. No matter how closely you try to replicate the actual exam conditions, you really never will be in the same mental state as that during the actual test. So if you already have a test date, you can still choose to go ahead and take the test. During the test, keep in mind the topics of the questions in which you face problems. This will help you prepare better for next time. It will also be a great test of your time management skills and performance under pressure. Of course, I will not suggest people to schedule the official test for practice but since you have already scheduled it, you can take a call whether you want to postpone it (and pay the rescheduling fee) or go ahead with it.