Thanks, @KevinJW for the feedback. I also have not forgotten your feedback to my query regarding experiences with ADX (and whether or not the per-channel gains were used or not).
It is probably a good idea to replace the LUT segment with a functional approximation and unclamp the bottom end, but I think it will be deferred to a later release since this was never on the list for ACES 1.2 and I would want time to test any changes that we make to this transform.
There has been talk of maybe starting an “ACES with film” virtual working group in the near future, and this transform would be well within scope for such a group to examine and suggest updates.
I’ve added an issue on Github to keep this logged for the next version.