Hello everyone,
Had temrerature issues with CRS305-1G-4S+IN and Mikrotik S+RJ10 transciever, so I also decided to add copper heatsinks as you were talking here.
With such heatsinks temperature of S+RJ10 transciever droped from 85C to 80C only.
So I have ended adding 5V USB-powered silent fan at the top of the switch and temperature is now 63C.
If not using optic fiber, fan is only good solution for 10G speeds with CRS305-1G-4S+IN.