the OEM exhaust flows quite well, the only restriction happens at the higher RPMs as the flow of exhaust gas becomes higher. A exhaust upgrade will indefinably help but you would not see much gain in power at lower RPMs.
There is a temp sensor on the condenser inside your dash where the A/C pipes connects to, if that sensor is faulty, the A/C clutch will not disengage, therefore the in-cabin condenser will freeze and air will not be cooled till the ice melts.