This is purely speculative, but I'm excited for the t8 4pc in part due to the fact that the instant heal could be considered direct and proc things as such. It could also proc revitalize. This is a real long shot, but it could crit (think regrowth without the cast), proccing NG and speeding up subsequent ring-around-the-rejuv's. If this were the case, crit would finally be a valuable stat. Speculation aside, I hope it makes it to live as it is currently as it opens up a lot of potential, especially for a 23/0/48 type build that revolves around rejuv for raid healing.

I have some 540 haste + 3% from CF. Which means that with NG I'm usually with sub-1s Nourish and when the bloodlust comes, my regrowth becomes a sub-1s heal as well and my HT is somewhere in the neighborhood of 1.5 secs or better, since typically I do get WoA, while boomkin auras are a bit iffy. This is mostly using "avoid the crit gear" idea, nothing crazy going on. So, honestly I think that for everyone who geared and specced for haste, this change is not that big of a deal. I see this being more of a deal for a person with 200-ish haste, but at that point the 20% will still give you a lot of haste - more than anyone with 600ish haste is going to get out of it.

I think this change is more to clean up the mechanics in general as it seems that Blizzard wants to get away from "-0.x secs on cast time" mechanic and go towards "+y % haste" mechanic. I think that's a good change because it becomes clear how this is calculated and it doesn't require a bunch of testing and reverse engineering to figure out whether this "-0.x secs on cast time" effect comes before haste, somewhere in the middle of the haste buff stack or at the end of it.

The duration change is actually a buff in my opinion. I don't think that my NG ever last 15 secs without getting wasted (i.e. I cast something silly like MotW and get a NG proc off it and there is nothing else to do for another 1 min.) It was only on the next spellcast, now if you get lucky you could maintain that indefinitely. Considering my normal crit is 9-10% and I get 25% on top that with either Nourish or RG, the crit on them will be still ~35% (closer to 40% if you factor in some auras, and such). That means that if you actually do some RG/Nourish spam to keep the tank alive or such, you may have that 20% nearly 100% of the time anyway, and in any case will count towards more spells than now. The main difference is that it will be up mostly when you actually really, really need it, as opposed to when you get lucky while doing next to nothing, then sit picking your nose for next 12 seconds to dump some healing touch or regrowth on someone who probably doesn't really need it that fast anyway. And if I want more crit options, I'll pick up a crit trinket or two rather than anything else.

Finally, the new NG will scale better because the 20% will count towards GCD as well, whereas right now you get -.5 secs from cast but that doesn't have anything to do with GCD. If the instant tick of 4T8 rejuvenation bonus is something that can crit, it could change the picture of NG completely to become somewhat of a low tier alternative to the high tier GotEM. Another alternative is that the we forget about haste on gear altogether, and gear 100% for crit hoping that NG will fill the haste needs. 20% is more than enough to bring GCD down to 1s with 5/5 GotEM.

If I understand your scenario correctly, Regrowth is your spell of choice. It's only .5 seconds longer, and gives you the HoT for SM or Nourish. Although I may not understand exactly what you're saying. Because if you just need a quick burst heal RJ to SM, but if you want to set up a "spam" on a raid target it would seem RG comes first.

Loatheb is another useful one.