Taking a look at a die shot of the GPU provided by ChipWorks, the site was able to identify eight GPU clusters. Notably, the layout looks a lot like that of the GX6450 with another GX6450 placed right above it.
Since Imagination doesn't have an official eight cluster Series6XT GPU design, AnandTech believes that Apple is modifying Imagination's designs to fit their needs.
Due to the fact that Imagination does not have an official 8 cluster Series6XT GPU design. While Apple licenses PowerVR GPU cores, not unlike their ARM IP license they are free to modify the Imagination designs to fit their needs, resulting in an unusual semi-custom aspect to their designs (and explaining what Apple has been doing with so many GPU engineers over the last couple of years). In this case it appears that Apple has taken the GX6450 design and created a new design from it, culminating in an 8 cluster Series6XT design. Officially this design has no public designation – while it’s based on an Imagination design it is not an official Imagination design, and of course Apple doesn’t reveal codenames – but for the sake of simplicity we are calling it the GXA6850.
Take a look at the die shot below or hit the link for more analysis of the new GPU.