...theoretically the car should lose torque down low because there's no back pressure in the exhaust.
Wrong. Back pressure is needed by the engine and is provided to it by the turbo. The exhaust length doesn't matter provided the bore is right to give the best flow rate. A side exit will be better than a rear as the bore can be kept at a smaller size than a full system meaning the exhaust gasses leave hotter and and faster. A side exit will also belch out a shit load of flame