I've heard that someone tried and lost 5whp from it. More than likely the reason for the loss of power is a poor p/p job. If its not done to perfection it can cause issues, and/or show a backfire in hopeful gains.
And yes, people do run the greddy s/c and i've seen one dyno of a greddy s/c with a full exhaust hit 131whp. Not sure who it was that had the dyno, just saw a sheet somewhere online.