Whilst this is good information, Having done extensive testing for a large customer with an >3TB database, we had found that a specific table was consuming most of the upgrade duration and as such having multiple threads was less helpful than having more parallel processing on an Oracle database. 

I would recommend all Administrators talk to their DBA's to better tune the CTAS values to ensure an appropriate settings are chosen. You can review the upgrade SQL commands log that is generated to see how long each transaction/table is taking to complete and use that to identify exactly what is taking longer than expected.

That being said, we got the upgrade duration from 10 days (unacceptable) down to about 11 hours (Manageable) doing the upgrade with the tuned settings. So adding this capability was really great for my customer.