It is very very bad numerical wise.
I did a sim:
With T194P: 933k
Without anyset bonus:834k
Assuming 2pT20 is 10% more damage on Obliterate: 849k (15k more obliterate dmg)
So do the math. 4pT20 looks pretty weak, like 446k dmg per free FS. Sims use obliterate every 3sec, so assuming you can use free FS procs without overcapping and off GCD, it is like 2 extra FS/minute. So thats the maxium gain of 4p. In reality it will be worse because using might lead to Breath getting shorter or overcapping ressources.
But just to the math. 446k*2/60 = 15k.
So according to this, changing from 4pT19 to 4pT20 (with same stats) is a big dps loss.
DPS would go down from 933k to 864k.
While T19 was a 11.6% dps gain, T20 would be a 3.5% dps gain. That's a ridicilously weak 4pbonus for Breathspec. T19 would be have 331% value compared to T20.
T20 is a ridivilously weak set bonus unless there will be more synergies. But T20 is far far away from not being a big nerf. I dont think frost will be viable with that T20 set bonus. Would be last at single target.
I believe we are getting closer to the idea of having to go Unholy. And if unholy sucks, well... time to reroll.
Just look where frost would be single target wise with 8% less dps.
https://www.warcraftlogs.com/statistics/11#boss=1842
In this example frost would go down to 766k, which is very low.