One can easily see the dynamic iris in action by selecting a dead input, an all black screen, and turning it on and off. The black becomes a deeper, darker black, improving perceived contrast.
I leave mine ON and have never encountered a real world movie/TV scene where it had any negative effect, however on some earlier pjs I've owned (not the 8000) I could audibly hear the dynamic iris turning on and off, a very slight fluttering sound only audible when running the fan in ECO mode during dead quiet scenes under weird/rare circumstances, e.g. a scene with strobe lights used in a dance club where the scene brightness is dramatically alternating between dark and light many times per second. It cause a faint, unobtrusive fluttering/purring sound.
In A/V reproduction accuracy, there IS no concept of "accounting for personal taste/preference". As art consumers we don't "pick" the level of bass, nor the tint/brightness of a scene's sky, any more than we pick the ending of a novel or Mona Lisa's type of smile. "High fidelity" means "high truthfulness", faithful to the original artist's intent: an unmodified, neutral, accurate copy of the original master, ideally being exact and with no discernable alterations, aka "transparency".