I think most skiers can learn to ski better as long as they are willing to take the proper steps. Trying to learn to edge change right at center line is not something that's going to be learned very easily at a skier's top speed or line lengths. In order to learn something new we have to be able to take some of the speed and intensity out of the passes so that we can completely focus on what it is we're trying to improve. I learned to keep my elbows connected to my body while I was skiing "reverses" where we would set the rope on -28', start at a speed of about 26 mph, then slow the boat down with each pass and spin on the ends. That's where core strength, balance, and technique are improved. Try running six or eight passes, slowing the boat each time, and no stopping. You'll learn how to absorb the wake with your knees, how to keep the chest up tall, and how to keep the handle low and tight. That back foot skiing, tilting the head and shoulders, and dropping the chest will not get you very far at 20 mph.