When you mentioned you have a 2-story townhouse with one system, I cringe. If your townhouse system is like most townhouse systems I've worked on, the duct work is virtually inaccessible and nothing can be done to balance the airflow. Is this 8" x 18" diffuser located in the sidewall where the indoor unit is located? I picture the indoor unit located in a small closet on a platform with the return air filter grill located under the access door to the air handling equipment.
Your statement about the HVAC sales person saying you
might resolve the low airflow issue with a 4 ton air handler also has me cringing. You cannot move 4 tons of air through 3 tons of poorly designed duct work. Of course, I'm making assumptions, but you really didn't provide enough information about the system nor did you include any photos with your post as required by the
forum rules.