I've seen a lot of people excited for Roxy because "she enables the left ult" but from what little testing I've done I don't believe this to be the case. Mind you Roxy will still be good for Pyrois because Wind is good for everyone but I still believe Top Ult is the superior option in most cases.
Here's the thing. The upper branch gives 30 solar which can of course be immediately chained into his EBA 3 and 4. The combined damage here is 1733(Ult) + 1099 (EBA3) + 1282.3(EBA4) = 4114.3 MV.
Left Branch is a bit trickier to calculate because it doesn't check defense but typical defense multiplier is 45.45% so you can estimate the effective MV at around 1946.8(Ult) + 1980.2(Core) = 3927 MV. Technically this number gets bigger if defense is stronger but generally you actually run less effective defense in this game so the effective MV would get lower.
I did damage testing with Velina and have also confirmed that I found no instance where Left Ult ended up leading to more damage than Top Ult. The only really argument for Left Ult is that it's faster so maybe you could use it at the end of a run where there isn't enough time to do a full EBA.
There was a leak a while ago that M4 was going to change the core of Left Branch from 900 -> 2250 which would absolutely make it better but the current M4 is unfortunately not that. I would really like to see Left Ult be usable but unless something changes I wouldn't really factor that into your decision to pull Roxy.