It definitely is engineering. I always thought it would only be a design thing (to look different compared to the regular).
But although both versions have more or less the same top speed (XF ~ 250 km/h, SRT6 blocked at 255 km/h) the SRT6 needs more downforce in the rear.
Due to the additional weight of the supercharger the rear is lighter. So that's the reason why this ugly wing sits on the SRT6.