| Re: square tubing?
Having round exhaust pipes provides the quickest exit of exhaust gasses possible. An ideal exhaust system is perfectly engineered, round of course, and fitted to the perfect length, which will promote quick exhaust exit, and exhaust scavenging which in turn provides more power. The idea behind quick exhaust exit is that the exhaust pipe should theoretically get smaller as it moves farther and farther away from the headers/y-pipe so it keeps moving quickly out of the exhaust system, resulting in a "sucking" action, if you will. Opposed to an over sized exhaust system that provides no low pressure to quickly exit the exhaust gasses.
This is why mandrel bent exhaust systems are advantageous and provide more power, because smooth curves promote quicker exhaust exit, not like square or rigid pipes which would cause too much reflection of the gasses.
Hope this helps. | 
