Air leaking from the AC system into unconditioned space will result in air being pulled into conditioned space from outside to make up for the loss of air. It will just seep in from the points of least resistance such as doors, windows and voids in the building structure. The air will drag in whatever heat, moisture, pollen, dust and other pollutants it comes across. I'd suggest sealing that leak, every little bit adds up. A duct leak of 15% of the conditioned air will result in a 25% higher energy consumption for cooling and heating.