Compression light in the O4S-stream (OPL4)

In the Go Figure Under the hood for OPL4/O4S there was an "open question":

"... adding compression could be one idea and a nice challenge."

Now that the MSXdev deadline has passed, I could not leave that one, un-investigated. I immediately thought of a way that could be a low hanging fruit. Could I make compression without sacrificing performance at all? Or at least keep the same requirement, make all tunes from Go Figure below 5000 cycles per frame?

It could be done with 21% size reduction with no performance penalty.

Check out more details on this page.