Compiling GHC disabling the type checker

Hi, I would like to write a translator which takes a Haskell-like language to GHC compilable Haskell. This Haskell-like language is not explicitly typed, and cannot have types inferred for it (rank 2 types may exist etc), however it is known that the program will not crash with a type error. All case statements are well typed, i.e. case x of {1 -> ..; True -> ...} will not happen, although types cannot be given to the whole program. How does GHC react to a really large number of unsafeCoerce's? Is this likely to destroy performance? Has anything like this been done before? I know that LML was able to "turn off" the type checker, but I guess GHC doesn't have such an option, because of its typed Core language. Thanks Neil