I've two machines on amazon . com ec2, one having a drupal installation and the other one with the mysql database. I question whether it's worth to patch drupal to activate mysql client compression. What are the caveats?

yes, esp. once the queries return 100s of text records. ASCII compression is about 90%. Should you came back 1MB of ASCII records, compressed can lead to 100k. Plus, compression can be achieved on-the-fly, so you will find a rise in latency by microseconds, but you will have less data to proceed the wire.