Pam,
I hope you don't mind, but, as I frequently use the over-emotional trait myself, I decided to run my own little experiment.
My sims were Test Subject and Control Subject.
The Traits were: Ambitious
Bookworm
Genius
Natural Cook
I left Control Subject with just the four traits and added Over-Emotional to Test Subject
Both Sims had "The Cullinary Librarian" as their LTW.
I played until both had completed their LTW.
I played both together and had them do everything simultaneously. They went to the library together, showered at the same time, grilled identical meals in the park etc. I also allowed them to complete wishes when they both had the same one. I cancelled all wishes that they could not both complete.
Result:
Test Subject completed the LTW on Week 1 day 5 @ 15:04 - at that point her happiness points = 49,112.
Control Supject Completed the LTW the same day @ 18:01 - at that point her happiness points = 49,280
at 18:01 Test Subject's points = 49,289
So, there was almost no difference in points between the two, but, Test Subject was a little quicker when reading cookbooks and recipes. I found this odd as Test's moodlets were slightly higher throughout the test, but, made no significant difference to the happiness points at the end.
The extra speed in reading probably translates to other tasks, but, I'll leave that to others to test. Over time, the saving could be significant. However, I am not sure now whether I will use over-emotional so much.