It's premature to blame the capacity issue on infiltration. Have the unit checked out, then take it from there. There are several different issues that can result in low capacity.
This. As much as I appreciate building science, the first question I would be compelled to ask the OP is if the upstairs system NEVER kept that area comfortable. If the answer is no, it once did keep things comfy, even on hot days, the system needs attention. If the answer is yes, it never has made things comfy, ever, the solution may lie between HVAC system design and the building envelope.
Electricity makes refrigeration happen.
Refrigeration makes the HVAC psychrometric process happen.
HVAC pyschrometrics is what makes indoor human comfort happen...IF the ducts AND the building envelope cooperate.
A building is NOT beautiful unless it is also comfortable.