Exhaust gases are two things, hot and high velocity, which makes them, once they have exited the pipe, a high energy, low pressure gas stream. They can provide thrust and also if exited in the right area they can assist air flow over or under the car. I would never have the exhaust just point out of the car at 90 degrees or down, maybe up if you wanted down force but to the rear is best especially if you can use it to assist your aero package. I think Blue's (Eric) comment about, testing, testing, testing etc. fits here.
If you wonder how much force the exhaust can develop, you should see the vids of the fuel funny cars that lost the headers on one side, it nearly blows them over backwards!!
Rex