Eu desisti de usar o modpagespeed para isso e usei o nginx
location ~* ^/proxy/s3/s3.amazonaws.com/(.*) {
proxy_cache one;
proxy_cache_key $uri;
proxy_cache_valid 200;
expires 30m;
proxy_hide_header x-amz-id-2;
proxy_hide_header x-amz-request-id;
proxy_hide_header ETag;
proxy_hide_header Server;
add_header s3_proxy_cache $upstream_cache_status;
proxy_pass http://s3.amazonaws.com/$1;
}