the_Wop, PowerRec is a default function enabled on all Plextor drives; it's always on since the first day you bought it. Nero does not 'enable' PowerRec since it's on by default. Nero does not 'turn it off' unless you choose to.
Why don't you try different media and see if you get the same results instead of blaming it all on your drive?
Remember one thing: Computer problems are never unilateral and if PowerRec keeps slowing your burn speed to lower speeds then you must be using some very cheap media with your drive.