One problem at a time is the key.
Bhai probably the fan clutch, any relay related to the AC system or the evaporator. My boss has this city 2006 immaculate condition he had the same problem turned out to be a leaking pipe from the condenser and a choked evaporator...
Bhai I would not recommend AC walas because last time I got my AC serviced my car blew up the very next day...
Yup performance can be improved. New sparkplugs, HT leads, new filters, transmission oil, carburetor tuning alot can be done... But if it is fine don't mess about it..