It is not 7990 but a custom card consisting of two radeon 7970
Its like Asus ROG Ares series.
They dont use 7970Ghz edition, they just use the regular 7970, so a 7970 Ghz edition will be faster and you can oc more on individual cards
As BigMack70 said it will perform better than the 690 as even though the 690 is 2 GK104's with 8SMX units each and it is slightly downclocked.
but if i were you i would choose 2 7970 GHz editions