I was faced with the same dilemma in an engineering class. Had several good friends that I had helped study all semester, and they were consistently doing worse on exams. With a curve in the class, and when you are leading by a significant margin over the next best grade, it is fairly easy to fix the curve without risking losing an A. The math actually works out quite well in favor of being able to do just that.
My friends knew I was doing well, but had no idea how well (people remember that you do well on tests but don't seem to remember specific grades, much less take the time to calculate anyone's average but their own). They also had no idea that I was considering the curve-fixing dilemma. I really wanted to help them, but felt that it wasn't ethical. My problem was that by throwing the curve intentionally you are removing the decision making process from the professor, who is more qualified and authorized than you to set the grades for the class.
So I talked to the professor. I explained that I knew I could throw the final and bump up class grades by an average of nearly 10 points. I also explained that I had an ethical problem with doing that. So we discussed the situation, he agreed that one outlier grade wasn't appropriate to set a statistical curve with (it is actually standard in most any field to remove outliers from data sets), and that my performance shouldn't hold back my friends if at all possible. I urged him to take a look at previous semesters and see if I was just doing better than normal, or if everyone really was struggling for whatever reason. He found that the class as a whole was doing about the same as every other year he had taught the course, but by using my score to curve he would wind up with grades about a half-letter below previous years on average. He decided to adjust his curve to take this into account. We all got what we wanted... my friends got the grade they and I hoped for, I got a really high grade, and my professor thanked me for having the honesty and ethics to come to him with my concerns.
This isn't bragging - just a class I happened to really enjoy. I've been on the other end of the stick more often than not. To the OP, if you've helped your friends you have nothing to feel bad about at all. However, I wouldn't suggest fixing the curve, if only for the remote chance that your prof isn't an idiot, figures out what you've done, and adjusts your grade negatively. Go talk to him. Perhaps your friends are just below average compared to other years. If so, maybe they deserve the grades they have earned? If not, maybe he will see your point of view and be a little more gracious with the curve. Either way, you will have made a genuine attempt to both do the right thing and help your friends out, and I don't think you can do any more than that.
