if my car is redline is 7800 and the engine can rev to 8400 can that slow me down in the1/4 mile and can it make me have a slower mph

 

yes and maybe no
the best way to find out its on a dyno
dyno ur car.... find out after what exactly rpms the power band starts to go flat..
also if u comparing ecu's or the problem is that the same ecu is redlining lower.... what i wanna say its that a diff computer mnight have a diff A/F map with a shorter band and the other one might have a more stretched version of the A/F map ..... wichever.... the engines power varies on where the A/F map.. and if the fuel maps its not doing anytrhing anymore and where the power actually starts to go down...