The engines push the aircraft through the air. It wouldn't matter how fast the conveyor goes, as long as the wheels are free spinning enough. No matter the speed of the conveyor, the thrust of the engines in the air should overpower the conveyor. The air would remain static to the conveyor. The engines would simply overpower the conveyor if the wheels would spin free enough with no friction. The engines move the plane through the air, they don't move the plane along the ground. The ground condition should have no effect on the engine moving air.