The main point of the Speed EVs is to get the jump on the walls before they can strike at you. What you're using Drain Punch for is roughly the same, but instead you have the possibility of getting around being damaged by an attack. In other words - you are able to preserve your health a little better. Obviously, there is the minor drawback of instances where a Pokemon possibly stays in on Conkeldurr and you don't recover as much health as you normally would have. Here's your examples:
Impish Skarmory
Drain Punch - 37.42 - 44.01%
ThunderPunch - 49.7 - 58.68%
Now obviously I know what you're going to say - on minimum rolls, this would fail to 2HKO Skarmory. So in Skarmory's scenario, you would have to use ThunderPunch when it switches in. Still, with SR + Leftovers taken into account, the minimum Skarmory takes is 90.12%.
But the main thing is that you're trying to outspeed walls / bulkier Pokemon for the sake of getting two hits in before the opponent gets in one. Yes, there are some inferior disadvantages to doing it, and it's fine if you're deterred by running Speed. Still, that's why it's only in AC and not in the main set - this way you don't have to play a mind game if Skarmory is going to Roost or if it's going to simply Brave Bird you into kingdom come.
Not really digging the Superpower suggestion, since it negates Conky's precious Atk boost after 1 use. I'd much rather use Hammer Arm for a strong Fighting alternative over Drain Punch.
On the bulkier set, I definitely agree about Hammer Arm > Superpower. The set doesn't really care too much about Speed except maybe to Facade Reuniclus and it sort of helps Payback a little bit more. The Def drop is also nasty.
On the faster set, I would disagree. First, Superpower gets the OHKO on Scizor after SR, so the worst case scenario is you taking a Bullet Punch. Still, let's think about it - Scizor likely isn't going to Bullet Punch a Conkeldurr unless it's fairly low on health. It'll probably U-turn thinking it's faster, at least on Choice Band. To put it into perspective:
Superpower - 99.7 - 117.15%
Hammer Arm - 83.13 - 97.67%
Superpower also does roughly 59% minimum to Physically Defensive Skarmory, so a Drain Punch followed by Superpower will sack it without a major issue, assuming Stealth Rock is in play.
Also the main point with ThunderPunch is mostly for the faster set - hitting Slowbro, Jellicent, and Skarmory with the same attack. Don't bother with it on the slower set since Facade / Payback hits the former two Pokemon harder and Skarmory has to play carefully around Drain Punch anyway.
