I have the stone you are looking at....But after discovering a steel - I don't use the stone anymore. Would think that the Grill Grates - turned upside down would produce a similar effect of the steel - which is it is a great heat collector. What thickness of crust are you aiming for?
The Emile Henry stone, being ceramic - tends to insulate, and "diffuse" the heat, and will cook a medium to thick crust better IMHO.. The steel - and my "bet" the aluminum of the Grill Grates - act as a collector, and being metal - transfer that heat faster. SO - you will get a better thin crust.
If you are looking at a thicker pie, and longer cook - the EH ceramic may do what you want. If you are a really thin crust guy...you need much more heat to produce the kind of crust combined with that glutinous "tug" of a good crust.