Again, it depends on the games and the screen resolution.
For example, a Galaxy S or Nexus S has a SGX540 GPU, with a screen resolution of 800x480.
Galaxy Nexus also have an overclocked version of SGX540, with a 720x1280 screen resolution. And even when the resolution is higher, the games still perform a tad better than Nexus/Galaxy S.
Now, if you want to compare Mali 400MP with SGX540, there will be a significant noticeable difference in gaming performance, with intense graphics. Say, NFS Most Wanted, Dead Trigger. With casual games like NFS Shift, you wont feel any significant difference in performance.
So NO, it cannot be 80% faster just because its dual core. Performance is calculated on the basis of Average results.
Which Tab is in question anyway?