Of course it's realistic.
In the approximately two months that I've been self-studying, I've raised my average preptest score about 10 points. On occasion, I've hit as high as 168. If I can find the cash to take a Powerscore course, I'm confident I can raise it an additional 5-7 points, if not more.
I was in a hurry to try to get in for Fall 2007, which severely limited my study time. There is no doubt in my mind that a solid course and a few more months of study will dramatically improve my score.