Yeah I’ll say I do agree the shift to critical overcharge is too sharp, you get penalised way too much for that last bit on the OC bar - perhaps reducing the threshold for the Critical Mass stacks a little would help with that, so you can get full benefit while having a little wiggleroom in which to juggle and manage your heat. That or the final stack of critical mass would need to come with a way bigger bonus.
But up until Critical Overcharge I think you’re exaggerating the downside of reduced AS on her defensive capabilities a lot. Talking Legend and Cata, even at critical OC, I can defend myself perfectly fine on BW and Pyro without Volan’s, even without ult use. It just requires much more careful timing and push management. I can’t speak for everyone - but neither can you without doing a poll or survey - however I’d take Deathly Dissipation over Volan’s if they were in the same tier. Granted, it’d be a bit weapon choice dependent, on some staves/melee I might take one or the other depending.
Also, with Swift Slaying you have outright 20% AS back, constantly with how high Pyro’s crit chance gets, that’s leaving out properties and talents if you don’t take any AS ones.
Beyond that, Pyro has way better venting potential than BW thanks to the THP ult. I vent all the time on Pyro to get more casts in, before Volans I’d sometimes vent for melee safety too if I had a staff with bad clear. It’s pretty easy to generate THP on her.
On top, most of Sienna’s staves have amazing horde clear. Only really bolt staff comes to mind as a problem if you aren’t somewhere where enemies are forced to come at you in a line, I can’t comment on beam as I’m not a fan of how that staff plays so I don’t use it myself, the rest I know for a fact have good enough horde clear or control that I’m generally facing very limited numbers of enemies at any given time. Coruscation especially on Pyro with Heat Sink and venting absolutely annihilates hordes and unarmoured elites.
Looking at all that, the slowdown is perfectly manageable. You make it sound like you have no options, can’t see ahead and vent before enemies reach you in melee if you need the AS over the crit, and like you’ll be facing the full force not softened up by staves.