I need to say that, years ago at CaliforniaMan, John gave me a free fitting session two days before the race. He was standing around idly and we got to talk about biking and before I knew it he volunteered to take a look at my position. I got a free Cobb fit, I could not believe my luck. Many travel half way around the world to meet with John.
Many people have commented on John's intuitive sense and his deep insight and I have to admit that I became a believer in those sixty minutes we spent working out details. My position improved dramatically. Unfortunately, and this needs to be said as well, changing one's position two days before a race is not a good a thing. I paid for it by barely being able to run.
Therein lie all the lessons of a good fit.
First, remember to make sure you are comfortable. Endurance races are long events and comfort is at a premium. Within limits of course, and a bike will never feel as comfy as a couch. But if you opt for a distinctly poor position you will not reap any benefits from it, no matter how aero or how otherwise "correct" that position is.
Second, never make abrupt changes, and especially never change things in the days before a race. Even if the changes make sense and are for the better. You do not want to race using a new position even if that position is orders of magnitude better than your current one. The same rule applies to all things new: don't race with new shoes, new cleats, etc. You don't need surprises on race day.
Third, fit is a personal thing. Some people are easy to please while others are never happy. Even some pros never stop "fiddling" with their bike fits. Eddy Merckx for one, was famous for constantly checking and fine-tuning his position. It is doubtful that such changes make any real difference and more likely these behaviors are just one way of dealing with pre-race jitters.
Fourth, almost any bike that isn't way too small or way too big can be tuned for a proper fit. If in doubt, smaller is usually better. It is lighter and stiffer and positions you lower so there is less wind resistance. All these properties matter a lot.
Fifth, unless you are a pro or have money to burn, you don't need a custom frame. Your body is far more adaptable than frame builders would have you believe. Remember that performance in endurance events is not limited by muscle but by your cardiovascular system. Talk about better power or better leverage is of very limited value. What matters most is not to compress your chest by pushing your diaphragm all the way up and to stay as aero as you can be while preventing it. Often the best position is a compromise. Too aero may lead to too much power loss.
Last but not least, a good fit is one that feels comfortable and looks clean. If your buddies tell you you look awkward, it is worth taking a look at your fit. Especially when you are slow or working excessively hard. Remember however, that some people always look awkward and that many have idiosyncrasies that deviate from the accepted "norm" yet they perform at a very high level. If that is the case, chances are you will be better off to keep doing what you have been doing all along.