What is the effect of no-current periods in commutation on output power of a PM DC motor?
What force would be contributed to the motor during those no current periods? If there are not many commutator segments, it means your getting further and further away from the magnet you want to attract to or repel from, meaning more of the current in the coil would end up as heat, Instead of significant rotational torque. (The force is proportional to the angle) 

Having the commutator segments too close also means a longer period of time where both segments may be powered, and as above, if your not angled correctly, your just loosing amperage to heating, not torque. I would guess the gap is fairly close to a half or a quarter of the width of the brush that makes contact with the commutator. 

Next up braking force, There is not much when the coil is open, If the coil is open, you get a lot of voltage, but next to no current, so not much power is actually extracted from the motor acting as a generator, equally why a properly sized resistor can brake a motor much faster than shorting out the 2 input wires, the braking effect is dependent on how much energy your taking out of the motors rotational inertia. lots of V but tiny I and lots of I but tiny V are not as much as the resistor that is at the "maximum power point"