How faster one can crack RAR passwords on AMD HD 7990 than on HD 6990?

We haven't done any tests with 7990 specifically, however AMD GCN architecture (which 7990 based on) is even better than previous AMD architectures for RAR password recovery.

Single 7970 shows speed somewhere around AMD HD 5970 and AMD HD 6990. So AMD HD 7990 (being 2x7970 in fact) should be about 1.5-2x faster for crack RAR password than AMD 6990 (depending on core speed).