Quantifying gains is beside the point. I don't think anyone became a great designer or a great programmer by only ever striving for "good enough". Yes, at the end of the day you are worthless if you don't ship, but you are half-worthless if everything you do is a hack.

Maybe for this case in isolation going the extra mile to this level of perfection is not justifiable, but if the designer is doing good work and meeting deadlines then you should keep your bean-counting micro-managers far far away. To attempt to quantify everything (even in programming) is to take the human element out of creation, and that runs a severe risk of throwing the baby out with the bathwater.