I definitely agree that unboxing can help a great deal with performance and space utilization.

However, as containers does not currently require any exotic extensions, I think that perhaps a type family -based generic map would belong in another 'unboxed-containers' or 'adaptive-containers' package (both of which currently exist on hackage), as it dramatically extends the language extension footprint of containers, taking it from something that easily runs across a wide array of Haskell implementations to something very ghc-specific.