It'd be interesting to see how well a monohull would do...
Difficult to say. The Groupama 3 is a giant trimaran (105ft), has a crew of 9 and was built for the job. To my knowledge there is not a giant monohull built for that purpose. But I believe that for equal sizes the multihull will be faster, but not as much as many people think.
I am more interested in small crew or solo races in more human sized boats
and the last one with top class racing monos (Open 60) and racing multihulls (Multi50) was the Transat Jaques Vabre (Nov. 09).
There have been 14 Open 60 and 6 Multi50. From the Open 60, four boats retired from the race, from the Multi50, two boats retired. It was a Duo race (two men for each boat).
The first to arrive was Safran, the Open 60 skipped by Marc Guillemot, then the inevitable CrÍpes Yahoo, skipped by Franck Escoffier, as usually the fastest of the Multihulls (only 12 minutes behind). Then arrived all the Open 60 (9) and only 3 days after the last one, arrived the second multihull. The last multihull arrived 11 days after the last monohull.
The Transat was raced with strong winds and stormy seas and while the monohulls could go full blast, the multihulls had to take defensive action, taking courses that put them away from the storm.
From the ones that tried to pass, "CrÍpes yahoo" make it through, "Actual" pitch-poled.
We will have to wait for the next Transats to see if these relations are true, but it seems to me that a 50ft multihull is a match for a 60ft Monohull, if the weather is not really bad. In very strong winds and stormy seas it seems that the Monohull has the upper hand and the multihull has to take big risks to follow.
Some nice videos of that race:
YouTube - Transat Jacques Vabre 11/11/2009 : La temp√™te approche
YouTube - Transat Jacques Vabre 2009: Revue des bateaux