One of my first jobs was siting modern roundabouts in Michigan. I helped site in 8 or so over a few year period.
Modern roundabouts have certain features to help the efficiency: relatively small radius for the roundabout, deflection so people enter the roundabout at a tangent, entering traffic yields to traffic already in the roundabout, and a mountable inner circle.
The roundabout the Mythbusters video is not a particularly efficient design. It operates as a circular T-intersection since you would enter the roundabout radially. Bump out the entrance with a little deflection, and you could improve the throughput quite a bit.