I have a 4ac engine. I also work as a mechanic 5 days a week. Anyway last year I totally re done my aircon last year. I had the same problem as you.
1. If your standing in traffic the aircon doesn't perform as well as if the cars moving. That means the compressor is weak and changing it will help.
2. There is a Black / sometimes Silver canister normally located in the front of the car (next to the radiator somewhere) On my 4ac its located directly under the coolant reserve tank. This canister should be changed ( if it hasen't been changed in the passed approx 4 years or so) to give better flow of gas i.e cooler air.
3. The condenser which lookes like a radiator but in fact sits righ in front of the radiator. Make sure there is no bugs / leafs stuck in it. Give it a spray with the hose.
4. Mare sure that when the aircon is switch on that both thermal fans turn on.
5. I had a 3rd fan installed on the condenser to allow better cooling of the condenser. The colder the condenser is the colder the aircon is.
6. And finally make sure there is a adequate amount of the proper aircon gas in your system, and that you have no gas leaks.
The two main that contributed to a better aircon were the type of gas in the system and that canister I was talking about. I would check these out first.
P.S The old type of gas is much better than the new type. The only thing is. It's way more expensive, but worth every cent.