We have a 5.8 in our group that races with this type of configuration. From my perspective, they are a little faster than the stock boats in light wind (more sail area aloft) and about the same in stronger winds. They still take a portsmouth hit though due to the fact that they are not using a stock mainsail (adjustment factor "MN").
Tim Dry
http://www.catamaranracing.org 
