I always assumed that with a high tailwind the ground speed would be higher, as the air speed would always be the same irrelevant of winds.
Some time ago on a KLM 737-700 we surpassed 1000km/h at FL390 which I thought was cool. Don't think I ever flew that high or fast short haul since. It cut quite some travel time off the AMS-ATH flight too!
http://upload.xandrios.net/1489136788_IMG_0289.JPG