RE: Mistral Naish 8'11 - any experiences?
You can probably squeeze a 7.5 if it's light. Steve is right, though, it's pre-wide style.
Positives: very fast, nice jibing when powered up, good jumping ability.
Negatives: less stable, doesn't tolerate bad footwork, very fragile.
For moderate wind, I really like this board as narrow-style boards go.
I use my 8'7" more as a pure wave board, and in fact the dimensions are similar to an Acid 7x (leaving rocker / rail out of it). I've ridden the 8'7" (which is substantially smaller) with a 6.5 and it's OK but really likes to be more powered up and I prefer riding it in 5.5 - 4.0; never really pushed the upper limit of an 8'11" since it was borrowed. Definitely a 7.0, probably 7.5, I doubt it'll be good with an 8.0.
Another board I really liked of that vintage / series was the Naish Fish. They were all great riders, but light and fragile.