SilverArrow20XX
Walks in the Light of the Crystal
That's definitely my impression.
There's some validity to the KHII-->Days order, since obviously that's the release order and therefore the order in which the story was actually worked out. And when KHII was originally released, they definitely intended the critical events that take place during Days to be mysterious off-screen stuff that you only experience in fragmented flashbacks. Days itself would then be the "backstory chapter" that fills in the rest of the holes (kind of like Birth by Sleep).
However, with the KHII Final Mix content added in, the Days-->KHII order definitely feels like more of a "complete package" from a storytelling perspective. You the viewer know the full story, so you get to see how those events shaped the characters and their actions in KHII.
So it really depends what you're looking for out of your experience:
Do you want to be dropped in to the story in media res and face the outcomes of events you aren't familiar with? Might make more sense to play KHII first.
Do you want to more of an omniscient view over how the story unfolds, including the full context and emotional impact of the events that precede KHII? Watch Days first.
I played them in release order originally, and personally don't think most of the mysteries in KHII really add much to the experience. In fact, I appreciate the characters and the weight of what's going on a lot more now that I'm playing through Final Mix for the first time after sitting through Days.
Both orders have their benefits. Honestly, I think completing Roxas's opener in KHII, then playing Days, then playing Sora's part of II is probably the best for impact purposes. The vast majority of KHII's impact that is ruined by Days is in Roxas's segment afterall.