3 observations about this talent.
1) PWS is a much less important part of disc rotation at 85. It is less appealing for raid spam because players have relatively huge health pools and are comparatively rarely in danger of dying; it is also a lower portion of your single target healing.
So, forcibly casting heals merely to reduce Weakend Soul (WS) could be a total waste, even a loss of overall throughput. At best it is less exciting than you'd think through wotlk mindset.
2) If it were worth stacking haste just for this, we'd see "haste plateaus," which shadow say for mind flays in wotlk: for set, exact values of haste, you can fit another MF between mind blasts, or could fit another heal/GCD to squeeze out an earlier PW:S. This is poor design that developers do not like, because haste had extremely diminished value for every single point except the point that raise you from one plateau to the next.
Further, more haste can paradoxically reduce the number of PW:S you eek out; exactly reaching the plateau is good, but further haste gets your "rotation" done earlier and you might start another cast (WS is still up), delaying PW:S. Casting exactly 3 MFs then MB: good. Casting 3, having more time, casting an entire 4th MF before MB? very very bad.
This is too difficult for a typical player to theorycraft, and too difficult for even great players who understand the theory to execute. Disc doesn't even have the option of clipping MF casts, so it would suffer even more.
3) Clunky implementation--yet another priest ability where the effect has a delay after the cast, and it has very negative impact on gameplay this time (see: Body & Soul, Evangelism stacks for other examples). 6.5s left on WS means you should be able to fire a 2.5s Heal followed by PWS as your next GCD, but you can't. You'll have to sit around doing nothing and wait for the effect to trigger before you can PWS, or start another (Heal?) cast.
That means that there are a huge number of scenarios where the talent essentially has no impact at all, despite spamming Heal on a WS target, because you end up casting Heal for 5s (eg) on someone who had 5s or less on WS anyway, or not casting at all (which is fine; throughput is no longer king but if you can afford to cast nothing, then spamming to get more PW:S is not very appealing either).
It also means you're encouraged to cast Heal immediately after PW:S, then fill with instant/GCDs while waiting for the triggered effects or last seconds of PW:S; casting Heal "late" in that rotation means the proc trips late. But Heal is least valuable in the GCDs immediately following PWS, because chances are at their highest that incoming damage gets absorbed instead. . .
Overall: This will probably be a nice talent to have when it just "happens" passively, and will be a way for multiple disc priests to finally "stack" in a raid instead of only being able to step on eachother's WS debuffs. I am not expecting it to ever be worth gearing or casting specifically for... though theorycrafting could prove me wrong! IMO, the reduced importance of PWS, the "haste plateau" effect, and the clunky proc implementation mean this talent will be "good" (at best) but never great.