I'm thinking of doing a little experiment with stacking steel & kiln on the weekend. But which staking order is the better one...steel on the bottom, kiln on top or the other way around?
Mike, other than possibly stress out your oven shelves, stacking wouldn't do anything for your setup. The goal for stacking is to recreate the effects of 1/2" steel with less weight for those oven owners that are in the typical 550 (+/-25) realm, without having to incorporate an oven mod. Once you have an oven that can exceed 600, 3/8" has no problem matching any bake time that 1/2" can achieve at 550.
In other words, you can have any NY bake time you desire (2.5 minutes up) with your hotter than normal oven and 3/8" plate.
You have yet to zero in on the precise temp for steel that will give you 4 minute bakes, but, with the pies you're making, I have a strong feeling that once you hit a 4 minute bake, you might find it a bit on the soft side, and that you might resonate more with your slightly crispier kiln shelf 5 minute pies. Even if you really wanted to hit that 4 minute bake, it would probably be easier to achieve with the kiln shelf, considering that's what you're more comfortable with. You're working at 575 on the kiln shelf but could go higher, right?
All the alchemy that we do boils down to typically anemic oven temperatures. If you're one of the lucky ones that can hit higher temps, the need for special stones fades away. In theory, if someone had an oven that went to 675, they could go to Walmart, get a 10 dollar stone, and, with sufficient skills/knowledge, be producing sublime 4-5 minute pies.