I have been distance racing on a nacra 6.0with the gps in Track mode for the last couple years and done a batch of speed testing. My best tracks are about 13.2 knots Upwind in ~ 20 knots true (flat water) Note that this is not VMG.
I am also a footer by habit.
Broad reaching under main and jib ~22 knots with a large crew

down wind with the spinnaker ~22 knots My observation is this is usualy as fast as you go with a chute. Your vmg goes up with windspeed because you just keep sailing lower

Last year at the Around Newport race My highest GPS reading was 24.8 knots. This was going dead downwind in about 30 knots true windspeed with the chute up. The boat felt like it was planing for a while. My crew and I were sitting on the rear beam with the jib over on the wrong side trying to figure out how the hell to take the spinnaker down without imploding. I remember the bows were clear of the water. Wave height was less then 1 meter.

On the other hand, I measured 52.2Knots on my DN iceboat going downwind in ~22 knots windspeed. To me this is scary fast.

I figure from practical observation a Nacra 6.0 can go ~20-22 knots reliably without crashing and burning as long as it is windy enought that you can twist the sails off and move the center of effort down lower.

Sail fast,
Take Chances.

Eric