Your question is not SDE-specific, but rather VLDB in nature, or Very Large Database. As your database grows larger...
and larger, it takes longer and longer to perform certain operations, such as take a full export of your entire database. To speed up the full export, look at the DIRECT path export. The other suggestions are already being implemented, like exporting to multiple destinations in parallel.
One of the things I question is the need to perform a full export. Why are you doing this? I sure hope it is not to perform a backup of your database. The export/import and Data Pump utilities are really not the means to back up a multi-terabyte database. The biggest problem is the amount of time it takes to perform that backup. Additionally, the recovery process can take days and days. You are better served by using RMAN for your backup and recovery needs.
Typically with imagery, the data is pretty static. I should know as two of my databases server imagery and are pretty close to 20TB each right now. I would be up the proverbial creek if I had to perform a full export of either of these databases. And even if I had the time to perform the export, I just do not have 20TB of spare disk lying around for my export destination. Since the data is static, I can back it up once with RMAN. I can then let RMAN back up only those changes to my database. This way, my backup window shrinks considerably.