Interesting results. Thanks for testing it. One thing that I'd like to point out that may not be obvious is that when a material like SiC has such high thermal conductivity, it can rapidly lose its heat to the environment, not just to the pizza. It's a lot like copper in that it can get hot fast but it can cool down fast too, making both materials (Cu, SiC) ideal for welding tips and gas burner nozzles. So SiC isn't the kind of surface material you want preheat, turn down the heat, walk away, and use to cook a pizza on a few minutes later, at least if you want a pizza to bake as fast as possible. In my situation, the insulation in my oven is 1970's standard and isn't very good, so the heat that the SiC loses during broil primarily escapes to the cabinet above my wall oven. Your oven may be much better insulated which is why your top is cooking faster than your bottom when you use broil. That and the fact you probably have a better broiler than I do.
Overall, given your setup, your outcome was probably as good as can be expected. Based on my calculations using your SiC distance and preheat time, your broiler/insulation is about 1.6 times more efficient than mine. If you raise it any higher, you will most certainly have to turn the broiler off before the pizza is finished. You could probably pull off a faster bake time than I have ever gotten.