Sure. The maximum velocity is apparently always Mach 0.86 for that plane. Whatever the altitude.
A bit above Mach 0.86 the plane is liable to break apart due to the stress created by sound-related shock waves. At Mach 0.86, those shock waves are the same whatever the altitude of the plane. (read wikipedia/mach number if you don't believe me)
According to the simulation software (which employs variables like thrust and drag) the engines have the capacity to reach Mach 0.86 at sea level and at 35,000 feet. But the plane can never exceed Mach 0.86 because of the risk of it breaking apart.
So it can always achieve Mach 0.86 whatever the altitude due to the plane's net thrust, but it can never exceed it due presumably to shaking.
Don't forget that a fixed mach number nevertheless results in a variable velocity depending on altitude. Multiply the mach number by whatever the speed of sound is at that altitude and you get an actual maximum plane velocity.
So at sea level, where the speed of sound is 661 knots, this type of plane would risk breaking apart at speeds above 568 knots (661x0.86); and at 35,000 feet, where the speed of sound is 574 knots, this type of plane would risk breaking apart at speeds above 494 knots (574x0.86).
A plane can also appear to go faster relative to the ground if the wind is behind it. So the air speed and the ground speed are usually different. A radar measurement from a fixed point will always have this issue. So if a plane appears to be moving along at 510 knots viewed from the radar station, but it has a tailwind of 25 knots, it's air speed is 485 knots. Since I believe the plane was headed West to East, that would be my estimate of the air speed of the plane (bearing in mind that wind speeds affect transatlantic flights by 5% in each direction).
Maximum Operating Velocities such as the ones Boeing provide are not relevant to Maximum Velocity caclulations. MOVs are a passenger safety and aircraft efficiency measure. They don't represent a maximum aircraft speed. They represent a maximum operating velocity which ensures that the aircraft is subject to low stress (and therefore that it can be maintained more easily and cheaply).
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