I do agree going from 4 shots to 3. that's the only nerf I see fit.
For the record the Calliope has always been a 3 shots to kill with 480 health. Recently it was changed to 400 to make it more vulnerable to counter/offmap barrages and the Stuka Dive Bomb.
As it's only really effective/wipey at close range to mid range, I would personally keep its lethality but further decrease its durability (to 320 hp and 110 armor) to increase its "shotgun" use from medium risk high reward to high risk high reward, as this decreased durability would make it significantly more vulnerable to counter attacks.