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The question banks are generally the same, yes, although some questions have been added, some have been edited, and some have been deleted in the past year. The standard deviation on our test is about 50 points; generally speaking, that means that most people score within about 50 points of their last practice test score on the real test. You can increase the accuracy of the test results by ensuring that you take the tests under officially timed testing conditions (including the essays - stamina is a factor!), and by taking the tests at the same time of day as you plan to take the real test.
One word of caution: CAT exams are really good for (a) figuring out where you're scoring right now, (b) practicing stamina, and (c) analyzing your strengths and weaknesses. The actual act of just taking the exam, however, is NOT so useful for improving. It's what you do with the test results / between tests that helps you to improve.
It's almost never a good idea to take a test more frequently than once a week. You're really just using the practice tests to figure out what you need to improve before you take another practice test - so most of your work should be out of whatever books or other materials you've gotten that teach you (a) the material you need to know for the test, and (b) how to take the test (the best techniques for answering different kinds of questions).