The GPU block in the A8 looks like 4.5x4 mm or 18 mm2, the SRAM is 2.2x2.0 mm or 4.4 mm2 and the CPU (I'm including what looks like the 2 separate L2 caches on the left side) is 3.6x3.2 mm or 11.5 mm2.
Actually my out of the blue layman's estimate (A7 total die estate 102, G6430 = 22mm2 meaning ~1/5th the total die estate) was around 17mm2 with the former rather dumb and simplistic reasoning.
Further dumb layman's speculation would say that the G6430 counts =/>280Mio transistors, while the GX6450 counts =/>390Mio transistors.
I can't help you much with the rest, but assuming that transistor density is close to reality the above for the GPUs might be close enough to reality.I don't think Chipworks ever gave the dimensions of the A7, but based on the 102 mm2 area and the proportions of the chip I'm using 10.5x9.7 mm for my calculations. That would make the GPU 4.3x6 mm or 25.8 mm2, SRAM 2.8x2.6 mm or 7.3 mm2, and the CPU 5.3x3.7 mm or 19.6 mm2.
So relative to the A7 the A8 GPU is 70% the size, the SRAM is 60%, and the CPU is 59% the size. Someone else can work out the transistor count changes, but it looks like the SRAM and CPU were pretty much straight shrinks, and the GPU grew a bit probably the change from G6430 to GX6450. So the question is still open where the new transistors went. Previously the CPU, GPU, and SRAM were 52% of the die in the A7, but now they are 38% of the A8.
EDIT: Is the secure element on the CPU? Could Apple be setting aside a larger portion portion of the die for the secure element/enclave for fingerprint data, credit cards, and possibly anticipating other types of secure data like health?
Since there isn't a dramatic increase in L3 cache I guess I'm back to suggesting the possibility of data compression in the memory and possibly flash with a hardware accelerator in the A8.
And in case anyone is wondering I am speculating a density of ~13Mio/mm2 for A7 and a ~22Mio/mm2 for A8.