Stacked cache is for high-end due to the increased cost. APUs relying on the iGPU are historically targetted at low-end. Doing an APU with stacked cache would be definitely possible, but it must be seen where the market is for such a product in the PC world, because enthusiasts will use a discrete card anyway, and most of low-end users will be limited by the iGPU FP performance. There are probably users who will love an iGPU and stacked cache, but it must be seen, if they are enough to justify the effort. We have seen that when there is an interest AMD is more than willing to create exotic products (i.e. Van Gogh, consoles' APUs).