Hi Old Horse,
Ken is correct, there are Formula and slalom race sails, then there are recreational sails.
What you really want on the Phantom 320 is something in the Free Race category.
IMHO, the Sailworks Retro or Severne Glide would be amoung the best sails on the Phantom
I've used both.
The Severne Glide comes in both 7.5 and 8.5m2.
The German Magazine Surf tested the 7.5 Glide and stated that it was as powerful as
most 9.0 m2 sails.
The Glides are exceptionally light if you put them on a 100% carbon mast.
The Retro has lots of low end power, and probably a bit more higher wind
stability than the Glide, but the Glide has probably the best light wind power
in that size range.
Other lofts "longboard" sails will also work, but the Glide has the most power and
is for sure the lightest when rigged on the right mast.
Hope this helps,