eSRAM bandwidth and DDR3 bandwidth can't be added for a simple reason, the datas have to go through the northbridge or the gpu memory system in order to acess eSRAM bandwidth. So the datas begin to move at 68 GB/s from the 8 GB DDR3 (assuming your engine is loaded and fully operationnal in main RAM (but that's a given))
Once they arrive in the northbridge, they need to be read at 109 GB/s at least one time by the gpu memory system,
Then the gpu can do.. his work.
THEN the datas are written back at 109 GB/s to the memory system and eventually you can read another small chunk at the same time and at the same speed, but only to the point that the eSRAM is discharged of its previous load (think communicating vessels here), and you hypothetically would not write some bigger buffers in a larger memory than 32 MB (modern engine want to do that, it will happen all the time), wich means you have to read from the eSRAM once again to place your stuff in the DDR3, back and forth.
That's a very long way to get up to speed... The first read of the datas will always be at 68 GB/s and to access more bandwidth it will always be from a tiny 32 MB chunk and it's always a two steps process, at least.
In PS4, you read your data from the one and only big 8 GB pool at 176 GB/s from the start, then write back at 176 GB/s once processed, in a large memory without limitation of size. Straight fast and simple.
