Craig, I use the term 'thermal durability' as a more concise way of saying 'resistance to thermal shock,' and when I talk about Fibrament, I'm referring to the material itself, not the aluminum clad grill version. Fibrament, on it's own, has very little resistance to thermal shock, hence the need for an aluminum heat spreader/deflector. I'm not disagreeing with you, but I think it's important to note that thermal durability isn't countless slow trips from 0 to 750 (or, for that matter 0 to 1500, which Fibrament can comfortably do), but, rather, the very quick trip that occurs when a non cladded stone is exposed to flame. I'm splitting hairs, but too many people see a connection between peak operating temps and thermal durability, where, for the materials we talk about, there is none. With aluminum deflection, fibrament becomes very durable in a grill setting, but the material itself is intrinsically very weak thermally.
As far as the Fibrament grill stone being the best option for a grill, it depends on the grill and on the intended application. Just like steel isn't a one size fits all solution for the home oven owner, Fibrament can be unnecessary in certain grill settings. In a non rotisserie burner grill, then Fibrament is one of the best choices, but there are quarry tiles that have even less conductivity than Fibrament. This lower conductivity makes them very thermally weak, so deflection is critical, but, in theory, a non rotisserie grill with the right quarry tiles can provide better heat balance than Fibrament. The only advantage Fibrament has over this scenario is that you don't have to embark on the difficult quest of finding quarry tiles or have to put together some sort of deflector.
As far a cordierite goes in a non rotisserie grill setting, it, again, isn't as simple, straightforward or as easily to set up as Fibrament, but cordierite + deflection, as seen in countless LBEs, can be just as effective as Fibrament. Finding the right deflector and positioning it is no easy task, so I'd generally recommend the Fibrament grill stone over cordierite if someone was shopping for a new stone for their grill, but, if they had cordierite on hand, I might recommend shopping for a deflector over Fibrament.
When you get into rotisserie settings... the choice becomes application specific. Neapolitan can't happen in a typical non rotisserie grill, but when you add the rotisserie to the equation, it's possible. For Neapolitan, Fibrament's low conductivity is excellent, as you found out, but, as I mentioned, the right quarry tile/deflector could improve one's chances by providing even lower conductivity/better top to bottom heat ratio. For NY, once you have all the necessary top heat from the IR burner, you can pre-heat a cordierite stone as low as you want.
In summation, just like there's no best stone for every oven, there's also no best stone for every grill either. Fibrament is definitely the best stone for a non rotisserie grill for the owner seeking the least setup hassle, but when you get into a willingness to seek out harder to find materials and/or get into NY style with a rotisserie, there's better options.