Remember also that the right mast for the sail makes a big difference.. I have a Gaastra pulse, but no Gaastra mast, and it is truely horrible. I've tried NP, Tushingham, and powerex masts in it, but regardless of even completely ridiculous amounts of downhaul (up to 3cm past spec) it is still crap. It pulls like a train sideways, but won't go forwards at all - exactly like a sail with not enough downhaul, only it has plenty.
But you're right - 9 times out of 10 a rubbish feeling sail can be cured by more downhaul.
Even just pointing out the rigging notes on the sail and explaining how to measure the correct luff length can make a big difference to someone who's struggling. At least by pointing out the manufacturers instructions, you're not just some random w@nker who's been out showing off and appears to know everything! Not that windsurfers are ever like that of course...

Now, how to help out the guys that show up at the beach every summer with an old raceboard, tie on boom and a floppy triangle sail? The kit obviously worked back in it's day, but I'm stuffed if I know what a correctly rigged triangle with no leach looks like!