A Mega Alakazam with maximum speed investment hits 438 speed. Mega Heracross whereas only hits 273 speed when fully invested. Because of Tailwind, the 273 speed would have been doubled to 546 therefore would have out speed you.
Now I haven't played competitive battling in a while and when I say I while, I mean since early December 2013 so feel free to disagree with my judgment and thoughts here. But base 75 speed for Heracross really isn't that impressive nor would fully investing in the speed stat really get you anywhere. Personally I wouldn't invest much in speed and after searching around on Youtube and Smogon, it seems people are being slightly more lenient towards running little investment in speed and more in HP. After all it does have the perfect typing for a physical tank, investing in HP makes a lot of sense. I've seen sets running little to no investment and some suggesting to run 96 to out speed base 86 Pokemon without investment which would bring it to 210. Should that be the case, Alakazam would out speed regardless of whether the Tailwind is up or not.
Should the situation arise, I would play it safe and switch. However if I am correct about people running little speed investment, I would try get some prior damage off on it and make a judgment as to how much investment it has in HP. Just don't make stupid gambles. On the other hand, should it have Tailwind support, it may not be running much speed seen as it wouldn't out speed much important and still loses out to a lot of scarfed Pokemon. Then again, this situation is unlikely to happen.