Sorry, it looks like TDE prevents effective backup compression: > Encrypted data compresses significantly less than equivalent unencrypted data. If TDE is used to encrypt a database, backup compression will not be able to significantly compress the backup storage. Therefore, using TDE and backup compression together is not recommended. This is from Books Online:
You might want to also try some 3rd Party backup vendor like SQL Backup Pro by RedGate. They provide different compression settings so you might see some improvement compared to default compression in SQL Server but generally compression looks for patterns to compress which you wouldn't see with TDE backups compared to regular backups.