"How do the new longboards like Phantom 380/320 perform in really light stuff against sailboats?"
they dont ....
sailed my old F-2 lightning against sailboats on a irregular course quite a few times last summer.
When the wind was light , there was no keeping up.
When the wind got to the point where i could plane on beam reach I could more then make up the time I lost during the windward beat.
BUT these races were not a true windward leeward course. The course was around existing government buoys and there were quite a few reaching legs .
Typically the sailboats will race, three beats to windward with two downwind legs. The downwind legs being straight downwind, no reaching legs.
And its the reaching legs are where a sailboard has an advantage , again in medium to heavy winds.