I'm the GM of a 25man guild, and tonight we snatched up the realm first Heroic Al'Akir kill...
... in the 10man version.
It's just silly, really, but then you have to ask "why not?" We've got about 35 raiders. We can bring a completely optimal class composition for the fight, and overgear those 10 in our normal raids. We skipped heroic Conclave for a week, since the gear there was pretty bad anyway.
Many people predicted that this would happen - 25man guilds would snag up the realm firsts in the 10man setting. And out of the 305 heroic Al'Akir kills worldwide, 274 of them were in a 10man setting, most of which were completed by 25man guilds who did it during off hours.
Sharing realm first achievements was a failure, plain and simple.
I really hope that 4.2 has split realm first achievements that are mutually exclusive. There's just no reason to continue forcing "10 and 25 equal equal equal equal" when they obviously can never be truly equal. Sure, they can be the same in difficulty, but separate simply cannot be equal.
I don't understand the desperation to call them "equal" anyway. Why can't things just be "different" and give the same rewards? 3v3 is very different from 5v5, which is very different from rated battlegrounds, and yet all of them give the same rewards.