Re: I always wondered ...
Maria doesn't use an Instinct. She uses a Youtek Radical MP (pro I think); she endorses the Instinct with her racket just painted to look like one.
As far as I know, Maria's Head racket is indeed heavier but is more flexible, so it eases the shoulder unlike Prince rackets which are known to be very stiff.
Faves: Sharapova, Hantuchova, Safarova, Goerges, Hardebeck