My thoughts: how much would the person lying down sink down into a 6-8" pad?
I have found lying down on thick memory foam mattresses uncomfortable because I am big and sink down too much, making turning over a challenge.
Whatever you add should be very firm in order to be comfortable and maintain the height needed.
2015 Rockwood A128S
2010 Rockwood 2604
2014 Ford F-250 CC SB 2WD
2008 Toyota Sequoia 2WD