As far as I understand (which could be wrong or changed), efficiency only comes into effect when the power level is set to 75 or less.. With greater returns the lower the power is set at..
So you would see very little effect from efficiency on systems with 70 power.. But quite a bit of an effect from systems set at 25 power.
On the other hand, performance skills add power directly to a system at all times (i.e. Weapon Performance skill-ups directly boost weapon power at all power settings)... Instead of just lower powered ones like efficiency.