There's a bit of a gray area in that optimizing time and attention management itself takes time and attention, and so your happiness and accomplishment aren't necessarily optimal in absolute terms after taking the time to work out optimal allocations. But that's a quibble; people very frequently engage in behavior they know at the time to be long-term suboptimal, and when we talk about things like willpower and akrasia we're primarily concerned with minimizing that sort of behavior.
With this in mind, I'd describe willpower as the component of your priority-evaluation algorithm that counteracts present-biased preferences. Hypothetically perfect willpower would mean no temporal discounting, although discounting future possibilities in proportion to probability of occurrence doesn't seem like a failure of willpower to me.
For the past few years my willpower has been steadily increasing. If it lasts and I use it to accomplish something noteworthy, I might write a post about it.
Anyway, what should a rational preference utilitarian with infinite willpower do? Assume that there are no negative effects (unhappiness, stress) with using this willpower, and that they can control their emotions at will.
Clearly they should work a lot more, not spend time on recreation (movies, TV, games), stand instead of sitting, etc..
What else? Should they listen to music? Should they keep their muscles flexed 24/7 ? What should they learn, where would they have the most relative advantage? How much time would be worth spending on social interaction?
I can figure out these things on my own, but those questions are important and good ideas are very valuable.