Gerard,
The JD 8100 griinder is a really neat setup - looks like it will chew up a big stump in minutes. The first issue is finding one, then the concern would be in rocks embedded in the stumps comprimising blade life, then last how would it work on 40" butts - I suppose they have a larger blade or make multiple "drills" but this is very cool indeed.
The other video appears to core the stump - maybe 18" - again it has its place I suppose but for big stumps its a multi core job it would appear.