I just don't believe this thread!
Experienced pilots thinking airspeed is in any way affected by turning up or downwind? Jeez! I've only ever before come across such gross misunderstanding among model flyers who remain earthbound and assign mass accelerations to their model rather than the air mass it is flying in!
Regrettably it doesn't surprise me at all. As well as model flyers its a common misunderstanding with hangglider flyers too. Sometimes, when teaching ex-hangglider flyers, its taken me several hours of demonstration to convince them, and then I'm sure they dont really believe the evidence of the instruments. As has been said, the trick to convincing people is go high, say 5000', so they cant visually reference ground features in the turn.
Another great misconception is what clouds do in the wind. You know those nice puffy summertime cumulus clouds? Did you know you can get out of the wind by flying upwind of them - they shelter you from it. Yes really!