Just a few points over charging is bad only because the condenser efficiency is designed to a exact charge, so on hot days there is to much vapor & it is unable to handle the change of state back to liquid, this is were the high pressure safety cuts out the compressor. On a cold day the system may work fine as the condenser is able to handle the change of state ie lower pressure with refrigerant in liquid form. Even with correct charge high pressure cut out can still happen on extreme hot weather days especially when car not moving relying on radiator fan. Car refrigerant charges are very small so it doesn't take much to over charge or much to leak so efficiency goes, all systems should be charged by refrigerant weight to system specification. Over the years compressor seals have improved but unfortunately for a rotating seal it leaks a very small amount of the product it is sealing to create the seal, this is why after a few years your car A/C can become less efficient & require the refrigerant charge serviced.