The Maillard reaction is responsible for the browning of bread, vegetables, meat, etc. The temperature of the surface has to achieve a temperature between 280 - 330 F (140 - 165 C) for this reaction to occur. There will usually be some evaporation of moisture from the surface of the product that will cool the surface. Thus, you need a grill temperature between 350 - 400 F to get the browning to occur.
Above a surface temperature of 356 F/ 180 C the Maillard reaction ceases and you start getting other reactions that can produce undesirable byproducts such as acrylamides.
Please remember that the effective temperature is the temperature to which the product is exposed. That may or may not be the temperature indicated by the grill thermocouple used by the controller. Check the temperature at the grate level on which the dish is being cooked and set your grill so that the grate temperature will be between 350-400F.