I don't know what to say. If you google "kephess calibrating shot" and look at any of the guides written months ago, they all mention that this must be interrupted to prevent enrage and most mention that it increases damage, damage reduction.
which was last modified in April.
Now maybe they have decreased the time between casts in 8-man because everyone has an interrupt now. On 16-man the cast is every 4 seconds. I can only interrupt every 4th cast (probably because of lag) with a 12 second interrupt. You don't need to get every interrupt, just enough to prevent enrage and get through the phase relatively quickly.