How much energy/prop efficiency does the e-volo copter sacrifice by having a smaller propeller?
What counts is the mass of air which is available for lift/thrust creation. Several smaller props would still offer about the same area for lift creation as a single rotor, so in this respect that design is not at a disadvantage. The smaller props will have to spin faster than a single rotor to have the same average dynamic pressure, and the lift distribution over the whole area covered by propellers will be less asymmetric at forward speed than it is over the disc of a single rotor.

What should reduce the efficiency, however, is the smaller Reynolds number of the blades. Even if the blade airfoil is carefully optimized for their small Reynolds number, I would expect that at least 10% more energy is needed for the same lift compared to a conventional helicopter. The smaller inertia of the blades, however, is a big advantage in control, because the smaller propellers can be spun up or down faster, avoiding the more complex rotor head of conventional helicopters. With a modern FCS the e-volo should be controllable by individually adjusting prop speed alone. The electric motors most likely are capable of short-term torque bursts which far exceed their continuous performance, so control responsiveness should be adequate.

The smaller moment of inertia of the propellers becomes a liability in an emergency, because the e-volo arrangement is less capable of an autorotation landing in case the energy supply fails. That is probably the reason why the e-volo carries a full aircraft parachute. On the other hand, many small engines and propellers certainly improve redundancy over conventional helicopters.

I also have to wonder about the mass of the massive framework which is used to mount the rotors. It is in the way of the propeller flow and should contribute a substantial fraction to the empty mass of the e-volo. I expect the designers use it to store the battery cells, but I could not find much about this on the website - which offers, however, a funny movie which nicely sums up the projected capabilities. Just don't get distracted while the e-volo waits for you in hover mode: The claimed flight time is currently only 20 minutes.

My verdict: It is less efficient than a comparable helicopter, but not by much. The two problems are the lower blade chord, which translates into more viscous friction, and the high mass and drag of the framework.