Paul,
I would like Jamie or Kev to confirm this but in my experience ther is very limited use for a 400 rpm which is why you see alot for sale at cheap prices. I suspect Kev will tell you it will have SOME use on stone used dry but realistically how useful is that to you. This machine will more than likely sit in you unit taking up valuable space, even if it doesn't owe you much wouldn't it be better to move it on and put the money towards something that will earn you money?
I won't be in the least offended if anyone disagrees with me by the way as I have a 400 rpm machine sitting doing nothing somewher so will be happy to find there is a use for it
Terry