I have raced cars with their exhaust directed downward and that ended before the rear axle. To LOUD! The whole car body becomes a sound chamber.

My current car has a 4-2-1 header into a glasspack muffler with a tail pipe that follows the stock configuration over the axle and below the fuel tank. It ends in a convential oval muffler (2.5 inch dia.) that had been modified to shorten the route the gases have to travel. I think it adds very little backpressure beyond what the glasspack adds and tones done the rasp you get using only a glasspack. I like the sound and its not going to damage your heading.

I once raced a friends car that had it's exhaust end just past the header, Tech didn't chatch it and it was only for this race. I was going to "fix" his exhaust after this race and before the next race as part of the payment to him for letting me drive his car. After 30 minutes my ears had a ringing in them for 2 days afterward.