Hi John,
There should be no surprises here, if you pass a texture in domain [0, 1] through the RRT + sRGB ODT you will approximately end up in range [0, 0.8].
The ACES system is meant to work with scene referred values as input, which means you need plausible reflectance values to feed your shaders. There are only very few things in the world that have close to 100% reflectance, e.g. Magnesium Oxide approximately 97.5%, Spectralon over 99%. Snow can reach 90% and is around 80% in Antartica.
So the question you need to ask yourself is what that texture represents, if it is meant to be lit and represent reflectance values, it simply can’t go over 100% and a practical limit is 90%.
Here is a chart with some examples values (RGB are the linear ones):
If you are doing physically based rendering, I think it is part of the game 
It is also possible to apply a rolloff after the inverse display transform but you will never get perfect result. Here is a function I use often for that kind of tasks:
Cheers,
Thomas