Some people seem to be surprised that a few guild are (at the time of this writing) 5/7 heroic in Firelands. This should not be surprising. The reasons, however, of why it took several guilds a bit longer to get heroic kills in Tier 11 are not so obvious, so I thought I'd point them out. First, people had to level from 80-85. Next, they had to gear. Some guilds such as paragon were going into BWD/BOT in wrath purples, and cata greens/blues, and perhaps a cata epic. Going from the upper 200 iLevel to a tier where the iLevel is 359/372 is quite big. In addition there were new mechanics added both into the boss fight, but also into the PVE scene. The talent tree was different, and rotations changed.
Going into T12 is quite different know. The upper guilds have people in 372/sinestra gear, not anywhere as close to the difference there was back in December. They've learned new rotation and talent builds, etc etc.
Thus it should be no surprise that guilds are already downing these bosses. They are, after all, some of the best of the best, and have no where near the challenges they had back in T11.
My guild went 5/7 normal mode the first week, while it took us much longer to progress back in T11.
As a note, my favorite fight so far has to be Beth'tilac. Climbing up to the web and all that is quite fun IMO, much more fun then fighting shannox...