Was doing Altairus yesterday and managed to blow out a full Divine Hymn - while standing upwind all the time.
According to my recount, it hit 27 times during those 8 seconds (which comes down to around +112,5% haste), doing a whopping 31k+ healing per second then . The "maximum of 12 targets" is not enforced, and I am not sure if this is a tooltip error - since you can heal at most 12 people anyway, unhasted - or that it's a real bug in that it needs to cut off once it has hit 12 targets.
Since it shares an identical ratio as Renew (12 sec total with 3 sec intervals for Renew, 8 sec total with 2 sec intervals for Hymn), it is also on exactly the same Haste table as Renew. In other words, at +12,5% Haste, your Divine Hymn will hit 15 times, instead of 12. Pushing for the next "bump" is, well, pushing it, since you only get that at +37,5% haste, or 4805 Haste Rating.
As an aside, I learned that the buff from Altairus is just that, a straight-up +100% Haste, or better said, equal to 12.812,5 Haste Rating.