I did a detailed study before I decided to become an engineer, therefore, it's an informed source.
Ah, but stress leads to blocks, mood destabilising and yucky tummy feelings. Not to mention it's negative affects on sleep and appetite, which work negatively with productive thinking. And while yes, most serotonin does exist in the gut for digestive purposes, it is part of the DOSE family and thus works in tandem with dopamine and oxytocin and endorphins, in the brain to make the happy happy moods people get.
Promise, stressed people do not work more efficiently than happy people.