It already compresses it all.

It sends 2 bytes per block so the game has a max of 65535 unique block types (although some blocks branch off into million for example colour block can have an of 32 million possible colours)

byte would be 50% faster at sending maps but limit the game to 256 blocks and its already using over 4,000 (not counting the colour wheel =)

map sending is not that big of a deal for example 256 256 256 takes around a minute. with progress bar 1% to 100%.

i suppose the sending could be made by faster by using alternative socket protocols for sending maps but for now its no problem at all.