I had a horrible experience with the 21" Turbines on my P85D and have been telling everybody to avoid them if at all possible. After paying $4500 for the "upgrade", I hit one pothole at about 30-40mph and lost two tires which ended up at around $1100 after towing. Then a month or so later I noticed one of my wheels was completely cracked. This wasn't either of the wheels that hit the pothole and in fact I have no idea how that cracked (tire was fine).
Rather than pay the money for the new wheel, I spent $1k on four 20" wheels and $1k on four new tires. I couldn't be happier (well, unless I had saved that $4500 up front).