"These issues are really only of concern if you're processing millions of records repeatedly, in which case you're probably going to transcode the records into a more efficient format. If people are interested, I can post some notes.
Also, in most setups, the binary format has so little entropy that it is faster to read gzipped data off storage and decompress than to read the raw data. It's nothing like the "compression opportunities" in marc-xml though.
It would be great to see some notes posted on the transcoding. Thanks!