Amlogic S912 launch has been delayed by a few months, and if we are to believe the data from GFXBench (test 1; test 2), the reason could be that they switched the design from a Mali-T7xx GPU to a more powerful Mali-T830 GPU.
S912 processor is still based on four Cortex A53 core @ up to 2.0 GHz like in S905, but the GPU will be much more powerful.
One person noticed these results and wrote an analysis and comparison (in Korean) against the Mali-860MP2 GPU found in Mediatek MT6755 SoC (Helio P10) SoC.
So while Mali-860MP2 is faster for all listed benchmark the advantage is not that great. His analysis compared benchmarks (read post for details) concludes that the GPU in S912 could be clocked at around 650 MHz.
Thanks to Adriano for the tip.