I think it's not exact losing power but shifting it out along the power band, benefiting the top end when the PE is used.
On a similar note, I experienced a lack of low end torque when I disabled the exhaust flap on my OEM exhaust(akin to the PE which has no flap) which lead me to believe the stock software is optimized for some sort of back pressure, - though that does not jive with conventional free flow wisdom of benefits for turbo motors. :dunno: