personally, i would not bother with the foam - it's been proven already that foam'ed cars fail too. it's not getting to the root of the issue - the rear subframe mounts need to tie into the frame rails. plus, if it fails, you cannot weld (or rivet, really) at that point as the foam is highly stiff, and flammable.
you can pull the left-most cap and feel down in there with your finger - you should feel (and see) the foam if the car has already been done.
peter
2004 330i ZHP
2005 330iT ZHP
2010 328iT M Sport