When talking about just a 3 Kt performance change, it is not something that is easy to measure.
It takes many flights done in controlled conditions (absolutely smooth air being one) in both configurations to quantify what actual amount of change it causes.
When the blended airfoil prop was originally being introduced by Hartzell, one was offered to Vans for evaluation. Quite a bit of comparison flight testing was done and in the end a speed increase of between 3 and 4 kts was determined.
Opinions, information and comments are my own unless stated otherwise. They do not necessarily represent the direction/opinions of my employer.
Van's Aircraft Engineering Prototype Shop Manager
RV-6A (aka "Junkyard Special ")