Engineering requirements require bolt depth to be equal to the width of the bolt. Any more is just a waste of the bolt.
The temperature an engine runs at is quite a bit less than what the muffler bolts are subject to. They are in the path of the exhaust. 450 degrees is not enough unless they are out of the muffler. Most gasoline engines have their bolts passing thru the the muffler.
Silicon glues have a much higher temperature rating and they have the advantage of you can remove them when you want to.