If you look at the tooltip of the buff while you have it active, you will notice, that Demon Soul Imp does not increase your crit chance but the damage of a crit. So the 1st crit from a non instant spell has not 100% mehr dmg but 160%. The second 140 and the last 120%. You still need to have enough critical strike chance that you can get these 3 crits during the 30s. Otherwise the CD does not serve its full purpose.