Pzwerfer is nowhere near CalliOP, and you know it.
For reference i'll say the Katyusha is fairly balanced and both Katyusha and PW needs it's vet1 abilities improved.
IB4 you tell me about unit HP for which i say i agree on reducing the health on the CalliOP on top of reducing it's alpha strike by spreading out the rockets barrage on 6x4.
IB4 you tell me teching cost. This is the only thing that makes the PW not "completely broken i need a hotfix 1 week later after patch" on 1v1 and 2v2.
Against garrison: in case a rocket doesn't hit the model but the building, it applies far damage to the unit inside multiplied by it's garrison modifier.
-Katyusha needs 40 rockets to kill a model.
-CalliOP needs 27 rocket to kill a model
-PW unit needs 4 rockets to kill a model.
Damage profile:
PW: 80/40/24
C: 80/40/12
K: 80/28/8
Alpha strike on both PW and C is equal. CalliOP has the extra utility of area denial and truck destruction through the sheer amount of rockets it fires.
ATM the suppression is over the top as it only matters if you are using a 2nd PW to secure any kill.