Have you taken any GMATprep tests
yet? Since those are made by GMAC, they are typically regarded as the most accurate predictors of actual GMAT score.
As to whether you should push back G-day is dependent on a few things. Do you need to complete the exam before any deadlines?
Sorry it took a bit to respond to your post, but HImba has it perfectly. Test prep companies in general, and Kaplan
in particular, are very good at making our practice tests as close to the real thing as possible--but if there is ever any doubt as to the authenticity of a score, the best solution is to take an official GMAC-made practice test.
As for your score increase, it's always very hard to speculate about how an individual student will do. But 60 points is a manageable increase in the month of prep you have--especially since, if you put your best verbal and quant scores together, you're already a few points higher than either of your scores currently indicate! Give it your best shot, and if you're still not scoring where you need to 8-9 days before your real test, then you can consider whether rescheduling your exam is advisable based on the status of your applications.
I hope this helps, and good luck!