Our 25 man guild went in with an average ilvl of about 504-508 (13/16H) and we 2-3 shot the first 5 bosses (even got the achievement from Totorus). Well the first boss does feel a bit too easy. We didn't even get to the 3rd Lightning storm before we killed him. So a guild who is still working on Garalon; or has JUST beaten him should be more than ready for the first boss. But after that I would advise not going further.
We spent more time on the trash to the second boss than we did for the trash and the first boss combined; if that says anything right there. As for the second boss, he felt about right we still 2-shot the boss. Granted we had 5 people dead going into P5 and were taunt kiting the last 10% of his health after he enraged but still more than doable. However I have been hearing that the debuff on the 3rd door maybe overtuned for 10-man so you will probably see some hotfixing on that tomorrow.
Third boss again 2-shot but the dps check felt just right for that boss. The only hard part was when Sul was empowered and dealing with the sandstorm. So the healing check on that maybe tight for some guilds.
Tortos we 4-shot, but part of that was getting the timing down. By the end of the fight we had so many turtles up that we got the achievement without really trying.
Unfortunetly I was not part of our Megaera kill so I cannot really comment on that, but we did 3-shot it and basically was just learning the order to kill him and dealing with the fire and frost debuffs.