There's a tradeoff of speed versus fuel burn rate. Normally the fuel burn rate goes up with velocity squared, while the distance traveled only goes up linearly with the velocity. However, the fuel rate goes up even more quickly as one approaches mach 0.9 and above. And of course as the fuel burn rate per distance traveled increases, your range decreases. Also, as the plane burns fuel it gets lighter and can go to higher altitude where it has a bit less air resistance and can cruise with a bit more economy.

So, there are certainly airframe limits (but they must be worried about mostly while descending), and there's a range limit, and there's an economy limit. I imagine that the airline company has limits on the speed because of the economy issue, though the pilot may get permission to adjust by 0.02 mach or so, which might make a difference of (0.02/0.82)*(8 hours)=11 minutes...

They can't really make up 50 minutes on an 8 hour flight...