(a) Planning a single engine ga you are 400 kg above the weight to start the Approach.
Weight is a certification - ie AFM - limitation. No flexibility for the pilot. Presuming you don't have an emergency requiring an immediate landing, the option is to burn down to the limiting weight for the airport. Planned landing overweight is no different, philosophically, to planned takeoff overweight ?
(b) Can you compensate that with flying a faster approach Speed like 20 kt more for go around?
You could ... if the AFM scheduled such .. which it almost certainly won't. Refer to (a).