Hello,
Choice scarf does not seem to work on Drapion when faced against an Excadrill because in two pvp battles, an Excadrill managed to outspeed my choice scarf Drapion when sandstorm was NOT in effect, and the excadrill was not scarfed either as it used swords dance then earthquake (Also confirmed it wasn't scarfed by messaging opponent). However, Drapion outspeeds other pokemon with no problem like Latios.