Cooking cornbread at a lower temp for a longer period of time can cause it to turn out very dry (personal experience due to test baking in the past). I use 3" deep sheet pans, for large orders, and still bake at 400-425F. I also tent the top with foil halfway through the bake to prevent the top from browning too much. With the extra thickness I add an extra 10-15 minutes (total of 30-35 min) to the normal recipe bake time but I also use the toothpick (or wooden skewer for the extra depth) test to tell when it is really ready. I also agree with "cowgirl" about preheating if using C.I. or you may experience stickage.
Do I smell SMOKE?????