From the 2011 Mustang owner's manual:
Sport mode is a function of AdvanceTrac stability and traction control, not the EPAS system. EPAS is adjustable separately on 2012+ models.
AdvanceTrac Sport Mode
The AdvanceTrac system provides an available sport mode on some models. This can be selected utilizing the stability off button as shown in the table above.
Sport mode is not intended for use on public roadways as this mode provides less AdvanceTrac system intervention than when the default ESC and traction control systems are on. Sport mode will allow more spirited driving while the AdvanceTrac is still enabled.
Sport mode is a function of AdvanceTrac stability and traction control, not the EPAS system. EPAS is adjustable separately on 2012+ models.