Brion Vibber:
The point of compressing the file cache is of course to save disk space. Since this is a legitimate thing to do, I don't think there's a need to remove that option.
Hmm, I assumed this was intended to compress the data that is sent to the client. Well OK, in that case I suggest to make the file cache work correctly in all cases, e.g. by sending uncompressed data to the output buffer when the PHP lib will take care of compression.
The wfClientAcceptsGzip() branches of the code seem to duplicate the functionality of ob_gzhandler, and can be removed without loss of functionality.
An optimization would be to solve it the other way around: decide per request to not compress with ob_gzhandler when the file is available in compressed form in the file cache.