It's really not that bad.
120% cost is completely unplayable I agree with you, BUT you can reduce cost in other ways and stay under 100% even with psychosis. In fact, the cost-effectiveness ratio of Psychosis is much better than what you would get from a psi-headband, and the dps increase is really big. It's additive 15% btw, it will put you at 20% BASE chance.
Using headband ratios (if you choose the manacost addition instead of the crit), you would have had to trade off 27% cost if you wanted 15% crit.