Great Question. At 400 degrees nearly all oils will smoke, giving off unpleasant flavors, including the fats in meat. Therefore, for best food taste and to get those tender juicy steaks and chicken, we recommend grilling at grill surface temperatures of between 350 to 400 degrees, with searing at 450 degrees.
If you are doing low and slow, 225 to 275 is best.
Keep in mind, that lid thermometer on your grill is not giving you the temperature at the grill surface. It is only measuring the air temperature above the grill. So if your like me and open the lid frequently, the lid thermometer is going to show a much lower temperature than reality.
We have a lot of professional chefs who like to sear off their cuts of meat at 800 to 1,000 degrees (NOT on a grill mat) for 45 seconds to a minute per side, then they transfer the meat to a grill mat at 400 degrees to finish off for the remaining 6 to 10 minutes. Talk about cutting a steak with a fork!!!
Just keep in mind that these are a tool. Have fun and experiment with them. But like any tool, it will not work perfectly for 100% of all situations. Happy Grilling!