The current overylay used by MA! is the biggest I've seen. It takes up too much screen space. Especially the Average fps counter. In my opinion it's not accurate and I never find myself looking at it.
Please consider adding the option allowing the user to change the overlay size to their liking.
Also please consider making the option for the user to disable average fps counter.
Take a peek at MSI AB's overlay configuration options. Now that's customizability.