With 250 haste, 120ms and just focusing on the gcd spark spamming my macro buttons i can pull it off. If i have to move sometimes to maintain healing range or want to actually pay attention who i heal outside my assigned targets, it gets more complicated and i think i would need about 300 haste to get it more stable there.
I do the raid leading in our guild, so some encounters don't really favour me tunnel visioning the healing part, but a good example where i didn't have to lead anything and could at least for a while LB 5 targets (tanks and 3 burns) was this
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I switch out higher +heal items for haste if i dont think i can maintain 5 gcd's, 150 haste or so is enough to make the 3gcd and regrowth rotation comfortable.