First, Chipworks released a dieshot of the new Apple A8 SoC (stored at archive.org). It is based on the 20nm fabrication process from TSMC, which they allegedly bought the entire capacity for. From there, a bit of a debate arose regarding what each group of transistors represented. All sources claim that it is based around a dual-core CPU, but the GPU is a bit polarizing.
Image Credit: Chipworks via Ars Technica
Most sources, including Chipworks, Ars Technica, Anandtech, and so forth believe that it is a quad-core graphics processor from Imagination Technologies. Specifically, they expect that it is the GX6450 from the PowerVR Series 6XT. This is a narrow upgrade over the G6430 found in the Apple A7 processor, which is in line with the initial benchmarks that we saw (and not in line with the 50% GPU performance increase that Apple claims). For programmability, the GX6450 is equivalent to a DirectX 10-level feature set, unless it was extended by Apple, which I doubt.
Image Source: DailyTech
DailyTech has their own theory, suggesting that it is a GX6650 that is horizontally-aligned. From my observation, their "Cluster 2" and "Cluster 5" do not look identical at all to the other four, so I doubt their claims. I expect that they heard Apple's 50% claims, expected six GPU cores as the rumors originally indicated, and saw cores that were not there.
Which brings us back to the question of, "So what is the 50% increase in performance that Apple claims?" Unless they had a significant increase in clock rate, I still wonder if Apple is claiming that their increase in graphics performance will come from the Metal API even though it is not exclusive to new hardware.
But from everything we saw so far, it is just a handful of percent better.