Moved into a new home and following some cooling issues on the first floor, a HVAC evaluation uncovered an undersized air handler that services the first floor (two zone system one for second. Basically more registers in the rooms than output from the air handler also some incorrect sizing of the ducts coming off the air handler(about 20% under what is required per the evaluation.)

I have a good professional working with me on this. He recommended upgrading the air handler, installing dampers and said the existing compressor was fine (one year old) I wanted to see if it is common to hang air handlers from the roof any more... the unit sits right top a bedroom. Additionally, how close do you size air handlers to what is required? Do you normally go with someting say 5 % more CFM than required or try to hit it exactly.
Anything else I should be doing or checking now? thx