Here is an old post from Rémi making comparison between a Serenity, a Phantom Race or an Apollo, all rigged with an 11.0 m2 sail.
Results in speed ranking :
5th August 2007, 09:48 PM
In speed only :
In 5 knots : Serenity > Phantom Race > Apollo
In 7 knots : Apollo > Serenity > Phantom Race
In 10 Knots : Apollo > Phantom Race > Serenity
All the best"
Hope this helps to answer your question about Phantom/Formula earliest planing threshold with a big sail.