I think its 500% better than the average golfer, and its harder and harder (but not impossible, see Mickelsons US Open final hole) to fool them into a mistake.
I know one pro who had the coach temporarily suspend his grandson from a college golf team because as he followed him, he was upset with his course management, bombing tee shots and second shots on par 5's that had no chance of getting home, but did have a chance of finding hazards, etc. "Defense wins championships" in all sports, including golf.
Over on Cybergolf - and I think I posted it here once - I wrote a piece on the value of strategy. While I concluded that strategy/course management could only save you one stroke a round, perhaps 2, four shots in a 72 hole tournament would cost a pro millions of dollars over the course of a career. And the competition makes the margins so small between golfers that its enough to make the difference at high levels.
Having said all that, it would be interesting to self study your self, i.e. play to the middle of the greens, wide side of fw, etc. for a full round, and go back out the next day and take normal chances that pros don't take to see how much it would improve the average score.