Not Roger nor heavyweight, but I have sailed a 380 Phantom Race, Serenity, SUPer 10'0", SUPer 12'6" and UltraSonic on lakes and light winds.
Agree with Roger : by design and after having sailed both, a SUPer 10'0" or 12'6" are for sure not high performance windsurfing boards, but are great boards for learning both SUP and windsurfing, however.
The large SUPer have nothing in common with a 380 Phantom Race for example:
1) the lateral stability is higher on SUper with an easier time to uphaul your sail.
2) the daggerboard is way easier to move in its pit on the large SUPer.
3) daggerboard and fins are way softer and shorter on SUPer.
4) no mast-track on SUPer, only 4 inserts on the deck to screw in a mast base.
5) flat and comfy deck that is 50-70% covered with an EVA foam pad on SUPer; tortuous deck covered with numerous objects (footstraps, rail overheeler, mast track and its pedal, daggerbord trailing edge and foot pad protection) on the 380 Phantom Race.
6) rounded rails on SUPer for easy manoeuvers and transitions; boxy hard rails on Phantom Race.
7) gentle and progressive acceleration until planing is reached on SUPer (with mast base in the aftmost insert); speed and upwind/downwind angle far superior on Phantom Race in both schlogging and planing mode.
8) entry-level skills required on SUPer; expert to racing skills required on 380 Phantom Race.
So for a heavy weight on lakes and light wind, the choice should be driven by the skills and the main programme/focus that is intended:
SUP + windSUP : SUPer 10'0" or 12'6"
Windsurf in planing conditions only : Go or large Futura or UltraSonic
Windsurf in gliding and planing mode : Phantom Race
Windsurf in gliding mode only : Serenity