If brake temps were a problem we used to put the gear down a couple of mins early before landing - the airflow soon reduced the temperature!
The 300C is a limit for the hydraulic pipes in the wheel well bay - Airbus have set a limit of 300C to avoid overheating these pipes. It doesnt affect the take-off performance or turnround times only restricts you on when the gear can be raised!
There are many ways of restricting the amount of energy that goes into the brakes most signiifcant of which is to land without autobrake (as per Boeing). The amount of effectivness this has depends on whether the brakes are carbon-fibre (likes lots of heat quickly) or steel.
Having said that the brake fans ARE very effective in getting the temp down to 250C (The temp on the ECAM is the core temp and 250 represents equivalent of 300 when the fans are switched off)