more grooves = less surface area = less friction and stopping power for a given pedal pressure. So you should need to push harder. Not a physics expert but seems logical.
Apply the same force to a smaller surface area it will apply more pressure than a larger surface area. I don't know what that means for brakes or if the amount on a pad is even significant BUT I do know this is the principle for 3, 4, 6 puck clutches vs full surface clutches. Using a less surface puck disc with the same pressure plate gives the clutch more holding power on the flywheel.