Has anybody ever tried to use a 12'6 (380 cm) windsurf raceboard, like a Mistral Equipe, as a race sup or a downwind sup? I'd be curious how they'd compare, since the widths (25-27") are similar and they have similarly boxy rails.

Hi, I tried to SUP on old windsurfs. Yes, length and width are close,  to try these boards helped me to understand the importance of shape. Double or triple concave make the board very tippy, the volume on a SUP is more centered around you...
Old windsurfers have a good glide, they help you to improve your balance, and they cost nothing, good for masochist who like to spend time in the water.

The Equipe XR version is 12'4" x 26" wide 12.6 kg x 260 liters. Not terribly heavy considering it's got a full on adjustable centerboard. Had one back in the late eighties or was it early 90's?  of course I didn't think to try and stand up paddle it. But I did take a kayak paddle out with it once in choppy ocean conditions and the biggest take away from the experience was how much the board shuddered when going over chop. Nothing like anything I have experienced on my SUPs.

I SUPed on a windsurfer one design when I started, and also tried a Mistral SST.  They have better glide than a surf style SUP, but certainly don't compare to a 14 foot race board.  A flat one design might have been better that the SST, since the SST had more nose rocker which shortened the waterline, and seemed to slap and slow down on even small waves.  In a rec race against surf style board you'd do well, but I doubt they'd keep up with a piercing flatwater board.  Nose rocker might help for downwind, but the lack of speed might making catching bumps hard.

Peter