But that 300kg upgrade will require additional equipment/modifications and/or adjustments to the performance graphs - that is what happened with the 139.
If the standard aircraft was fit for the additional weight increase then it would have been certified as such in the first place.
As Non- PC plod says - overweight is overweight and at the very least you need to assess how much and for how long and then carry out inspections and perhaps calculate time penalties for critical components.
What would happen if you crashed during a PC1 departure because you were outside the WAT graphs? What would your insurance company say and how long would your licence last - remember ass, tin, ticket SAS?