This is more an extended comment than a full answer. 

There are a lot of ways to do the migration:

1) exp/imp is a good good way if it is possible between this different version. I think the main reason why export import is slow is that there is only one job who does the work. You can accelerate the export/import if you group your tables and do the export/import of this groups in parallel. Also index creation and constraint enabling  can be taken away from the import and accelerated. This needs a lot of preparation but for a 6G this seems a possible way. 

2) Transportable tablespace possible in 8.1, so you can upgrade you database to 8.1 dan do the transportable tablespace export/import. 

3) finally you can do the upgrade from 8.0 to 8.1 to 10.2 to 12.1
http://docs.oracle.com/database/121/UPGRD/preup.htm#UPGRD12359
http://docs.oracle.com/cd/B19306_01/server.102/b14238/preup.htm#i694345


I think all these methods are possible for your database (6Gb, 12h downtime) and the method you choose should be tested thoroughly.

You should not forget that besides your Database your clients must be upgraded and tested, too. Here Oacle describes the major upgrade steps:  http://docs.oracle.com/database/121/UPGRD/intro.htm#UPGRD52705