I have dabbled a lot in flex... I also have a 912 FA with mottishaw's Spencerian customization... this one has the most line variation but the feed still has trouble keeping up with ink, only a problem if you flex too much though.

Also in my collection, multiple Noodler's, ahabs, nib creapers, a Konrad, and a neponset.

I also have Pilot Falcons in extra fine and medium.

Also, a vintage Wahl-Eversharp and a vintage waterman.

Compared to the other pilot FAs, the 823 FA is so far the most reliable, with far fewer issues with railroading and providing a consistent ink flow.

Compared to the Falcons, there is a lot more flex (Falcons aren't really flex nibs of course) and flexing is much easier.

Compared to Noodler's, the line variation is better, the pen is built much better, and it writes perfectly without needing any tweaking. Of course it cost like 5x as much so this should be expected.

I prefer it to my vintage pens for its modern filling system and ink capacity, however the nib on my Wahl Eversharp is still one of my best flex nibs.

To answer your last question, unflexed, yes it is probably something like a Japanese fine. It is very springy and flexes with almost no effort.