As discussed on the ghc-devs list, GHC does not specialize functions with implicit parameters. It is no surprise that GHC does not specialize the implicit parameters themselves, but given a function with a type like

foo :: (?foo :: Bool, Show a) => a -> String

there is no reason not to specialize on the Show dict.

This is an easy fix—the specializer can just treat implicit parameters the same was as ordinary function arguments—so I’ll open an MR with a patch.