Broiler pre-heats only 'super heat' the top of the stone. Pizza bakes with the heat in the entire stone. I've seen temperature readings done, and, generally speaking, as the top of the stone gets hotter the temperature of the bottom of the stone drops.
You can pre-heat the stone with the bake function and then, before baking, try to pump a bit more heat into the stone with the broiler, but it's not going to give you dramatically faster browning.
If you want a darker undercrust in a 550 degree oven, you'll need a more conductive hearth. 1/2" steel works well in this regard, and 1/2" aluminum is showing promise as well.
What style of pizza are you trying to make? What kind of flour are using?