Yeah, lately I've been doing "nearly-politan" pies with the Fibrament at 550 for about 90 mins, with some top broiler action toward the end of the bake.
So steel sounds intriguing as a way to get from nearly-politan to the real thing.
Biz, I'm being a bit pedantic here, but, when Toby (Infoodel, Foolishpoolish) coined the term 'Nearly-politan' it wasn't in the context of being 'almost Neapolitan,' but, rather, in the context that it was pizza that had every characteristic of a Neapolitan pizza, but baked in a non Neapolitan oven. He used this term as a means of addressing the concerns of those members who felt that if a pizza isn't made in a WFO, it can't be considered the 'real thing.' I'm completely oven agnostic, and I'd like to say, since the time that the term was coined, most of the membership has taken on a more oven agnostic perspective as well. For me, Nearlypolitan, going by Toby's definition, is the same as Neapolitan.
All that pendantry aside, if the 'real thing' is your goal, steel might play a means towards that end, but only if you've got a strong enough broiler to give you top Neapolitan leoparding in less than 2 minutes. This is not a large number of ovens. By my accounts, I guesstimate that 1 in 300 ovens can provide sufficient top heat.
Now, if your broiler does fall short and if you want a sub 2 minute pie without a great deal of leoparding on top, but with a very Neapolitan looking undercrust, then steel might work for that, but you've got to have an oven that will reach at least 600, with 625 probably being the sweet spot for 1/2" steel. If you're hitting 90 seconds with some undercrust color right now with fibrament, you might very well be reaching 625 (or even as high as 700), but I'd take the temperature with an IR thermometer first before you purchase steel.
If it turns out that you're 550 dial oven is actually only reaching 550ish temps, then your best bet for a NP looking undercrust would be 3/4" aluminum.
I'm not sure that an undercrust leoparded only pizza is all that worthy of a goal anyway. With the lack of heat balance, you might end up with a gum line. I generally advise people seeking Neapolitan in a home oven to check their broilers, and, if their broilers can't cut it, go NY (or Neo-NY). If you want to take a photo of your broiler, I can tell you if there's hope it can do NP.