"I don't mean to be disrespectful, but I don't understand this statement at all. To me thats like saying you'd find a turbo or super charged engine less effective than the engine by itself.'
The purpose of using +P loadings in the past was to insure expansion when the bullet impacted things like leather coats or heavy denim. But modern bullets like the Speer Gold Dot, Winchester PDX-1, And Hornady Critical Defense have made +P ammunition obsolete. +P adds additional wear and tear on the firearm that isn't necessary. Or in your car comparison, it would be the equivalent of driving a Funny Car to the grocery store to go shopping.