I had talked to SVT few times about this and they said its is 18% They said they know the car has been tested. I know some of you may disagree but I lean towards what SVT told me. I also had talked to the engine plant at Romeo.
Interesting reading is the performance engine books like Sean Hyland. The hp numbers he gets is high but the thing I noticed like many of my friends that have their engines dynoed. They dont have water pumps on, alt and other items and show me sheets of high #'s