How can I tell apache to generally get the .gz static files available and only compress files when there is no .gz file available. xenforo.js is 4% larger when dynamically compressed by apache than with 7zip, this is not so much. But there might be an additional speed difference if apache does not need to compress static files.
# AddEncoding allows you to have certain browsers uncompress
# information on the fly. Note: Not all browsers support this.
# Despite the name similarity, the following Add* directives have nothing
# to do with the FancyIndexing customization directives above.
AddEncoding x-compress .Z
AddEncoding x-gzip .gz .tgz
# If the AddEncoding directives above are commented-out, then you
# probably should define those extensions to indicate media types:
#AddType application/x-compress .Z
#AddType application/x-gzip .gz .tgz