Some people do improve that much, some people don't - so it is possible, but not everybody does it. (Not even most people - a 700 is the 93rd percentile, so by definition, only 7% of people score that well.)

If it is going to be done, then 3 months should be enough time.

If you studied thoroughly the first time around and have already seen a decent improvement in whatever your starting point was, then you may need to take a course or do private tutoring in order to see another improvement of 80 points.

You definitely brought your score down by having to guess on the last 5 questions. You have to fix your timing issues or you definitely will not score a 700.

I'm not familiar with Powerscore. I recommend taking tests once every 2-3 weeks and analyzing the results of each test extensively to learn about your strengths and weaknesses and make your game plan for the next 2-3 weeks of studying before you take another test. (I tell my students to take at least 4 hours to go over a test, if not longer.)

Good luck!