Unfortunately, i've come to the conclusion that the modified Deni is OK,but not great for pizza.
The problem is that even if you preheat it for a long time, and get it to 750-800 deg, all that heat is lost when you open it to put the pizza in. The bottom stone stays night and hot, and so the bottom chars nicely, but the top isn't exposed to the super hot air as the hot air is lost.
I still use it, as it does as good to slightly better job than a regular oven and preheats in 15 minutes instead of 2 hours heating the house..but it isn't the solution to our oven woes. (In my opinion).