I have never heard this about the Posi-Temp and cannot really imagine that it is true. A clogged showerhead would perform the same function and I have never heard of this condition causing cartridge damage.
You could make your own flow restrictor. If this is a Moen shower head, the factory installed flow restrictor is basically a thick washer like thing that has holes to limit the amount of water. Alot of people take them out. Remove the shower head and see if it is there. If it is and the flow is still too much, you can remove it and make one out of rubber, plastic, aerator pieces, etc. with smaller holes or add another (thin) restricter on top of it. If there is not one there at all, you can either make or buy one. Moen will probably send you one if you e-mail them. If you make one, you want to make sure that the edges and all holes you drill are smooth and free of debris that can clog up the even smaller holes of the shower head. If you make one, though, you can experiment and get it perfectly suited to your taste.
I commend you for your water conservation. We were discussing the horrendous (and IMHO extravegantly wasteful) amount of water that some systems with multiple body sprays and shower heads use on another forum just today. I really commend you for your water conversation efforts, what ever your reason.