It's not that the stock system is that efficient, but without better breathing there's little to be gained by dual exhausts. As you increase airflow with an extra carb, cam or head work, the motor needs more exhaust capacity. But making HP is not the only reason to add duals - you'll love the sound, and it will sure FEEL like like you have more power. Your concern for cold weather starting and driveability are well-founded, though. If you do split the exhaust, be sure that you retain some provision for heat to the intake.
