Assuming that I understand the way the mod works correctly, it gives +80% additive base damage per unique status effect currently on the target. That being the case, it will be inferior to a maxed Primed Pressure Point until three different status effects have been applied. Why does CO seem to be preferred when it just seems to be not as effective unless you're also running a status primer of some kind?
Depends on the weapon/frame in question. Some need more than others, and some builds are also far more expensive than others. For most prime frames and weapons which come with a preset polarity or two, I find 1-2 forma to be more than enough the majority of the time.