“The recall is for a failing ECU that may stall the car. It’s a safety recall. I haven’t done the recall because I figured I didn’t really need to and am a bit worried that new ECU might degrade it’s performance.”
So you have a choice of having the "safety" recall or not because you fear it may degrade performance and you have chosen to ignore a safety recall in favor of some unknown degrade in performance?
I would guess that it is equally possible the recall might improve performance.