First, Airbus A320 doesn't go beyond 35,000 feet and don't compare it with A380, that's a super jumbo. Second, Airbus officially declares the max. speed of A320 as 850 km/hr so high probability is there that your GPS is showing some wrong readings+ I have traveled in all India based Airlines with posses A320 and during my journey in Pilot's Cockpit announcement have always heard that Air craft is flying at a speed of 800 km/hr.