Well, actually, only the Buick 215 is American,,,,,,the specs for the Rover are GM (as is/was the tooling) but the process they used for the block in England is better.
GM used to pour the alum in around the steel sleeves,,,,while the Rover has the sleeves pressed into place. The Rover set up assures that there is no "shifting" of the cylinder and therefore has better cyl wall square.
This is a great little motor. I have one on the bench right now with a pair of very rare 64 buick 300 alum heads. These were only available for one year in aluminum and they have larger ports, and valve seat capabilities.
I was going to use it in a Manta Montage kit that I now have for sale. (to help pay for the frieght of my new GT40 kit)
The MBZ alum V8 is quite a bit heavier,,,,but the 5.0 in my 92 500e made 326 hp in stock form and has wide torque curve.
With a few mods, this engine easily makes 400hp, and can idle almost as smoothly as the stock unit.
Of course none of this matters since my 40 will have a small block ford in it. (hopefully with 8-sinister looking stacks in the rear window!)