Hopefully they have improved the fit and finish of the undertray... mine was an early one. I would recommend using it as it does make things more solid and keeps everything smooth underneath (better aero?). The splitter can certainly be used without it...Looks great thinking of getting this splitter now that @FeralIndustries industries completely drops the ball on what they do (zero communication). However the under tray extension looks pretty cheaply made. Would you recommend getting the undertray extension? Or do u think it's fine to run without it.