In general the limiting factor is hull speed. Multi-hulls have a hull speed, just like monohulls, the constants in the formula are just a little different.

For a displacemt hull:

hull speed ~= constant* square root of the waterline lenght

That constant is proportional to the ratio of the waterline length and waterline width. If you make the hulls long and thin enough then they start acting like foils and anouther set of rules take effect.

Based on GPS logs over the last few years the maximum speed:

Hobie Wave ~22kt (very scary, wants to go in every direction)
Custom 14 _~24 kts (wants to pitchpole)
Tornado ~26 kts (wants to pitchpole)
Marstrom A ~28 kts (stern squats)
Hobie 18 ~24 kts (rudders cavitate)

These speeds are probably +/- 1-2 kts, they are just something the analysis programs spits out. I have only really looked hard at the 14 data.

A really cool test would be to test the top speed of your boat and then stick your rig (mast and sails) on an iceboat and test the top speed of your rig.