http://www.haskell.org/ghc/docs/latest/html/libraries/base/src/GHC-IO-FD.htmlを見てきました。そこでは次のように述べています。
-- We used to use System.Posix.Internals.dEFAULT_BUFFER_SIZE, which is
-- taken from the value of BUFSIZ on the current platform. This value
-- varies too much though: it is 512 on Windows, 1024 on OS X and 8192
-- on Linux. So let's just use a decent size on every platform:
dEFAULT_FD_BUFFER_SIZE :: Int
dEFAULT_FD_BUFFER_SIZE = 8096
独自の dEFAULT_FD_BUFFER_SIZE 値を変更できますか?