If you want to cover yourself legally, then get yourself a CASA or local regulator approved design engineer.
They will take the current aircraft performance data and create new performance charts for the slope and or wind corrections you require. This will then be submitted to regulator and once approved can be inserted into FM for company operations.
In our organisation, we check pilots into every strip greater than 5% as routine.
Above 5700Kg, as stated, you may be prohibited by regulations and or aircraft manufacturer.