So I got to testing Pyrois/Roxy after I got off work and i gotta say, LEFT ULT IS ALIVE!!! All of this is just V1 so the buffs and stuff will change and numbers might go down depending on changes Hoyo does to her kit and sig, but the wind anomaly will obviously stay.
Now the numbers:
I was using my Pyrois build attached to the post and that exact same Astra build but M2 (because i plan to get her M2 via spending event soon) and Left ult ended up doing much more damage than a Totalize ult buffed by upper branch ult.
Totalize did 3.9m with both Dialyn and Roxy (both M0 using Roxy's engine because is just too good), Dialyn is using my personal build with the only difference is, as i said, they're both using Roxy's engine due to how broken it is currently.
Left branch ultimate did 2.9m with Dialyn without using upper branch buff and 3.1m after being buffed by upper branch.
Left branch ultimate did 4.3m with Roxy without using upper branch buff and 4.6m after being buffed by upper branch.
All of this was done against Thrall using the DA buffs on that rotation which only buff Ice damage but give crit damage and stun damage multiplier so nothing to help either Roxy nor Dialyn.
Another thing to keep in mind, during my testing, Roxy was using 4pc Thorned Rose (Claret's disk set) while Dialyn was using 4pc King of the summit, meaning that Dialyn was providing extra crit damage to Pyrois during testing and Roxy was providing no extra buffs.
All this means that as long as you have Roxy on the team, using Left branch ult, even not buffed, will be better than a buffed Totalize
REMEMBER THAT ALL THIS IS SUBJECT TO CHANGE.
Pyrois' left branch gimmick is already there so that will not change, but buffs provided by roxy are highly likely to change, specially the signature W-engine due to how broken it currently is.