A body/brain that is not under stress from sound or heat will go faster over the time period of the race. If your head really likes the car shut off at the end of the race, chances are the exhaust is too load for you to make good racing decisions.
IMHO the pipe stopping under the middle of the car just makes the car a sounding chamber. The power increase might be nil over the turn out to the side,in front of the tire.
As an old guy that did not wear ear plugs sooner,and cant hear sxit now, put the pipe out the side and/or wear plugs.
MM