1) As the rope shortens, the angle of the rope steepens at the widest point of the turn. The greater this angle, the greater the force that the boat's pull imparts attempting to straighten out your ski...or to move you back to center.
2) the speed and cadence is significantly different as you shorten the rope...it gets much faster
3) the required path (to get around the buoys) becomes ever more exact as the rope shortens...leaving much less room for error.
4) The transitions going from pull, to pre-turn to turn etc, have to be much more precise, and occur much quicker as you shorten the rope.
If you try shortening the rope while free skiing...you will notice the incresed speed; but, from 15 off to 28 off, it might even seem to be easier. You may experience less slack, and even a reduced effort (due to reduced drag on the ski). That said, it is a bit of an illusion, as you can turn and transition very patiently. In the course, it is a very different story.
After 28 off...it will become harder with each shortening...but still, nothing like when you are in the course.
Anyhow, try it, it can be a lot of fun.