FWIW, I posted a ticket:
https://ghc.haskell.org/trac/ghc/ticket/9214 

Together with the following issues it would make a lot of data structures lighter in terms of memory footprint:
https://ghc.haskell.org/trac/ghc/ticket/7647
https://ghc.haskell.org/trac/ghc/ticket/3990