Thinking this one through... (Bear in mind that all Seneca turbo charged engines are fitted with CSU: assume constant RPM.)
Retarding mixture -> reducing fuel flow -> better fuel/air mixture -> higher energy exhaust -> faster turbine spinning -> higher turbo compression -> higher intake air density -> more fuel -> higher fuel flow.
But I think that chain of logic fails at the second last step. You get more power as you retard the mixture, certainly. But I'm thinking that if you reduce fuel flow by (e.g.) 2% in step 2, it might increase again by (e.g.) 1% in the second last step.
Overall and as far as I know, FF reduces when one retards the mixture under all circumstances (barring a broken component). It's the power output and EGT that behave in a more complex way.