if you hit a target at 100k hp or higher, it will always hit the hard cap, which is like the vast majority of pve enemies
based on your testing i also think your right that infused is likely doing nothing for the enchantment outside of the cooldown reduction
Decisive is considered pretty bad. It doesn't boost your ultimate gain by 40%, it gives you a 40% chance to gain 1 ultimate each time you gain N ultimate.
Your main source of ultimate is gaining 3 ultimate each second you're actively in combat, so Decisive only adds .4 per second to that. Of course, if you have a specific build in mind that took this into consideration, go ahead! But generally for builds, there's always something better.