I think 172 Speed EVs or whatever is needed to get the jump on +Nature Base 100 is the minimum so you can Brave Bird Mega Kangaskhan before it can Sucker Punch you. Max Atk is obviously needed as well. Of course you can run bulkier sets with Charti Berries and stuff, you should just be aware of the Kangaskhan stuff when doing so. Most smart Kangaskhan will just Return anyway because they can take a Brave Bird and because they would know about the mechanics but it's something to thing about.
Agree with this spread, but partially for different reasons. I think the biggest priority bracket tie between those two is probably Fake Out vs. Quick Guard, which is a move I really think people should be using if they're using Talonflame. The utility is a lot more practical against a good player than Tailwind, so the only real viable alternative is like Will-O-Wisp, Protect, and U-turn in the two non-STAB base 120 move slots. Spending a turn on a Pokemon as frail and weak to common metagame threats as Talonflame to setup with like Tailwind or Swords Dance is going to end up with you down a Pokemon immediately way too frequently, and recovering from that against a good team is pretty difficult, so QG is usually something I think people should be defaulting on their sets and sort of requires this speed point. In the WoW scenario it also ensures you can burn it before you get attacked, though unless you're predicting a Sucker Punch (which is a mistake like drug duck mentioned) that's probably not your best play.
Going a little faster to creep the Liepard thing might be worth it more to creep other Talonflame who think that is important rather than for Liepard itself, who isn't very good or popular, though.
EDIT: Also, since I read the OP a little more carefully since I posted: I'd be really careful about trying to EV Pokemon to OHKO things that are faster than and do massive damage to them, you're building around a scenario where a lot of things can go wrong. This is especially true because while I'd say you're usually safe assuming the EV spread is somewhere around the 172-whatever 2 over Liepard is Speed/252 Attack/4 Def and SDef/rest in HP range, you're always going to find players who are running weirder spreads... which is playing with fire when you're fighting a counter.
Scott Glaza. 2013-15 VGC commentator. Twitter
Check out pokemon.com/strategy for official strategy content and coverage of the Pokémon Video Game Championships, Pokémon TCG Championship Series, and Pokémon Global Link tournaments.