demoing is different in different places...
there's a store near my place and this is how the demoing there works
you get a racquet for 3 days, you pay 5 dollars. Anyou demoing money you pay will count when you buy a racquet from that place.
There's tenniswarehouse.com you can try. Demoing is free. You can get 4 racquet at a time, keep them for a week. YOu just have to pay shipping.
Anyways, I dont believe any pro would use a sub 10 oz power racquet.