As I've stated a few times before, Cultists should not get <Heretic Astartes>. That right there will curb most of their power. I would almost say they shouldn't get Marks either, despite this being something in the fluff, only because of the interaction with Stratagems. Overall though, the problem is the vast number of Stratagems which as a result encourage CP farming as cheaply as possible to open it up; this is why you see the "Loyal 32" in nearly every Imperium soup army. If Stratagems and CP were rare things that you could use maybe once per game, maybe two at most, it wouldn't be a problem. But as it stands you are rewarded for gaming the system to have cheap CP batteries to fuel the things that really want to use them.
I think GW made a big mistake putting so many rules behind stratagems. It's what led to the issue we now face.