An ATI Air Transport International Boeing 767-300, registration N313AZ performing flight 8C-3402 from Seattle,WA to Anchorage,AK (USA), was in the initial climb out of Seattle's runway 16L when the crew stopped the climb at 5000 feet reporting flight control issues. The aircraft returned to Seattle for a safe ILS landing at a higher than normal speed on runway 16L about 20 minutes after departure. After landing the crew requested emergency services to check their brakes.