Distribution of heat using a silicone oven tray
Silicone can be made to conduct heat well, but I doubt that's the type on your sheet.  A lot of the browning comes from infrared (radiated) heat that the metal sheet is great at emitting.  

Silicone will likely buffer the radiated heat and not allow the browning effect (Millard's reaction) the same way steel would.  My guess is you'll get more browning on the tops and sides of the foods (from the oven wall) than browning from the bottom.