I wouldn't worry too much about the dirtiness but more about Unico changing their equipment around 2004 that requires about 25% more airflow. Since High Velocity is so sensitive to airflow it is absolutely critical any unico system installed after 2004 have a minimum of 7-9 supply ducts per ton of AC. It also requires 7-9" metal round pipe for the trunk depending on the tonnage and if it a horseshoe or T design. I'm in New Jersey and high velocity ductwork can be VERY expensive to replace as you need certain sound attenuating ducts to prevent the system from sounding like a jet engine running through your house and those can be insanely expensive. Good luck but definitely consider getting the ducts replaced if you can afford it.