Shelf weight isn't necessarily total weight divided by number of shelves.....they are also rated but the info is sometimes hard to find.
Yep, I have plywood and a layer of foam on mine too.
As Jeff mentioned, check the fit and don't forget to include the frame in your calculation. Mine overhangs the end by a teeny bit (maybe a half an inch on each end) so I extended the plywood and foam to accommodate. Not all style racks will let you do that though and it's not ideal.