The "imagecopy" plugin in Volatility 2.4 does not decompress hiberfil.sys files from Windows 8 machines, at least in the tests that I have tried.  In most cases, I'm getting identical files out, which means that the hiberfil.sys wasn't translated into a native physical address space, which suggests it's not supported?  I have also tried using the Moonsols Windows Memory Toolkit which claims to support Windows 8, but that software seems to fail as well.