Hi Aixsponsa,
The answer is in my post that was actually entirely dedicated to exactly same question - http://www.checkthishouse.com/5750/20amp-or-15amp-gfci-outlet-20amp-breaker.html . I highly recommend to read the whole thing but the short version is:
You should not install an outlet receptacle rated higher than the branch circuit (including the circuit breaker protecting it) it is attached to.