The performance calcs for the Duchess, IIRC, give about 15 or 16,000 as the maximum 'cruising' line. Can't recall what the 'service' or absolute figures are - would need to check the books - but it's not really relevant for this discussion.
What is important here is that mid channel when one donkey goes tilt - I've got another one! That's where the benefits are. A single may be faster, easier to manage, cheaper to operate but here's the real benefit - when my engine fails my DRIFTDOWN is significantly higher than yours!
Thats what I call a performance benefit.
(And I'm not too sure I want to go long distance over water or over mountains in a Cirrus at the moment!)