Thanks NSEU.
It was pretty much just my own curiosity - but I was also thinking in the context of a passenger initiated evacuation when (if) FD crew realise what's happening with "less than" the ideal time to wait.
Was looking at a YouTube video of the GE90-115B attached to their B747 "test bed" - they had the other 3 engines shut down and it was holding it's own - then later on the ground blowing 400lb rocks around - guessing that it's one engine you really don't want to be too close to even at ground idle (probably quite a few you don't want to get too close to, but you get my point hopefully).