These numbers really depend on the compressibility of the content, compression scheme and the type of batching used. The metadata overhead is fairly minimal. LogDevice allows you to configure this on either the client, sequencer or rocksdb level.
Is some form of compression enabled by default, without having to tweak options?
No, compression is disabled by default. You can enable compression either on the storage layer, or by enabling batching and compression on the sequencer, or by using the buffered write API with compression on the client.