Haskell's module system isn't very advanced. In fact, it about as simple as it could get. This is pretty well known known and there are proposals floating around to add more advanced features. But there generally isn't much need and, as you mention, adding things like module type parameters prevents many cross-module optimizations (which GHC does quite aggressively).

In practice, it doesn't seem to cause much of an issue, but this may just be down to how Haskell tends to be used.