Roblox renders differently indeed than blender, usually I bake my metal map in blender by baking the node attached to the metalic value as a diffuse value, then you use this diffuse map for your metal map on roblox, depending on your lighting settings you can alter how the metal looks, or you can increase it manually by making the map more bright using image editing software, same thing for roughness, just trial and error until you get close to it