How is calliope OP if you take everything in the context? It has been nerfed already. It is the most expensive rocket arty in the game, needs a doctrine to be picked and is almost the entire reason why the doctrines exist in the first place. With the indirect fire nerfs that USF will have next patch, USF will already be in a really bad state and calliope will be pretty much used alot more to fill the gap of having no reliable blob handling unit.
Btw, panzerwerfer which is a unit you don't need to pick a doctrine for is getting a low angle barrage ability which has almost the same wipe potential as calliope has in the next patch