Well actually a cache would really help, you could save a ton of memory bandwidth by prefetching assets into the cache and having the framebuffer there as well. Xbox One developers have low-level control of 32MB ESRAM so they would have to specifically target it to get the benefit - I don't think games have been optimized at that level yet.
Another point is that the Iris Pro cache is simply a victim cache for L3 and AFAIK developers have little to no direct control over it. AMD might emulate this approach for higher end chips but I think it would be unnecessary if they simply adopted a quad-channel DDR4 interface.
Originally Posted by NaroonGTX
The X1's performance has already shown that their EsRAM doesn't do much at all for the performance. Games are still in lower resolutions than their PS4 counterparts and of noticeable visual inferiority as well. It doesn't magically boost performance in any meaningful way.
DDR4, especially once higher clocks begin getting reached will be enough for the iGPU, especially if they make tri- or quad-channel boards.