I just installed a Carrier 58MVC and their 16 SEER heat pump. I am using the Infinity control that is mounted on the middle floor. My middle floor and basement cool perfect. However, if I go upstairs, I get cooked. Average is about 28C in each of the 3 bedrooms. Currently, my furnance is pushing about 1000 CFM through the ducts for both main and basement floor. This unit is capable of doing 1400 CFM. Here is what I am thinking of doing:

I have full access to the attic on the top floor. I was thinking of installing ducts in throught the attic to each of the rooms. Then, I was going to run a single line from outside down to the furnance in the basement. I am thinking going outside since I don't want to rip walls to get up to the attic. Once, I get this done, I am going to zone the upstairs using a Infinity Control zone control.

I would like people to comment on the pipe I need to install outside to get up to attic. What issues am I going to have?