The sail area for the phantom is 84.5 square feet. The Sunfish is 75 square feet. This is because the spar and boom length is a little different. The sunfish is 13'8" The Phantom is 14' . Not much difference but this is applied to the whole luft. of the sail. For a casual sailor 10sqft isn't significant, but if you wanted to race or really feel what the phantom can do you need to match the sqft.
The spar and boom are the same diameter so the sail rings for the sunfish would work instead of the pocket design of the Phantom.