I don't think the powercore 4 would be a real good one to shorten, it's tuned to give top end gains. Shorter pipes tend to give more low end power, so who knows what would happen, it could throw everything off and lose power all around. I'd leave it as it is.
Generally on a 4 stroke shortening the pipe ruins the entire flow and you really could see a decrease all around. And from experience shortened mufflers are loud as ****.