Hmm. Very closely equivalent to "Whenever a creature blocks this, exile that creature" (flying first strikers can block it, and only get exiled after they deal their damage I guess?)

Yeah; that probably does earn it its huge casting cost.

Funny thing is they don't even get exiled if the damage is lethal.

Really? That's interesting. I was under the impression that the effect would work before state based actions took place.

SBAs check first, then triggers check, but even so that's only to put the trigger on the stack; they still need a period of inaction from each player before they resolve. Here the creature will be destroyed if it took enough damage, then the ability will trigger anyway but find nothing to do.

'If... instead' clause should do the trick, though.