It would perhaps be more 'realistic', and certainly scary and challenging, if human Psi/MC Strength and Skill were capped much lower than aliens, for example, capped at 50 each. Various mods allow changes to skill caps. You could simply sack anyone with Psi/MC Strength higher than 50. What would be cleverer would be a mod that either generates human Psi/MC Strength in a lower range (0=50 instead of 0-100), or that applies a 50% penalty to Psi/MC success calculations for any XCOM-controlled units (humans, or human-controlled aliens).