-Tailwind doubles the speed of your team for two turns
It lasts three turns excluding the turn that you set up Tailwind.
Regarding Foul Play in OO, generally you'd be using it on Psychic and Ghost-types who all have a lower Attack stat than Tornadus given its high base 115 Attack, so you'd probably be better off using Crunch, even with a (-)ve nature.
For the Acrobatics set
-Crunch deals reliable Dark damage, inflicting more damage than Acrobatics to Ghosts and Psychics once Flying Gem is used up.
x2 Crunch deals 160 base damage, STAB Acrobatics has 165 base damage. I would only mention Crunch in OO.
With Defiant, you could go one to say that Tornadus outspeeds and OHKOes the most common users of Intimidate (Hitmontop, Salamence, Gyarados) and more (Krookodile, Scrafty (which I expect to become more common next year)) after Intimidate (leading to +1 Attack) even if you've used up your Flight Gem.
You should also slash Substitute with Taunt, making the option of Substitute + Protect together in the Acrobatics set. It's very helpful for stalling out Trick Room whilst providing the benefits of both Protect and Substitute on one set. It was used commonly in Nationals and Words during VGC 2011. This should obviously only be done with Prankster.
I agree with you about splitting the Defiant and Prankster sets. As you said, they play differently, which merits the split. Defiant is an anti-Intimidate Pokemon with less focus on team support, Prankster focuses more on team support.