I can see the M8's usefulness for Airborne and its huge. They essentially have a MBT that is small enough for the 82nd to carry around with them on a jump into hostile territory. Lovely for the Airborne!
Now, lets focus on the rest of the picture. The M8 is useless to non-Airborne units - why have a miniturized version of the M1 Abrams when you can just have the Abrams? Its not any faster, has less firepower, less armor, it's an automatic loader and has a smaller main gun. So, there's no good reason to deploy it with anything but the Airborne.
Now we know that the UK's Scimetar tops out at 80kph, so we know that 73kph -- 45mph -- is not some unbreakable ceiling that no tracked vehicle can surpass. We know that a tank can potentially go faster than the M8. The United States just hasn't made one yet. I imagine that much has to be sacrificed, but consider -- right now the United States doesn't need another MBT nor do they need another all-in-one, yet they most definitely COULD use a fast moving tank with reasonably strong armor and a good mix of firepower. If you want to focus on speed and very little protection, the Humvee works. Then again, the Humvee isn't that great at running over/smashing through small trees and brush -- making its own path where a path doesn't exist. Maybe I'm crazy, but I'm perceiving an unfilled role here. That or the Brits are just stupid for coming up with the Scimetar --- unlikely.