Isn't the problem here the fact that the iOS platform uses a unified memory architecture? This means the GPU uses the same RAM as CPU. The problem lies in way iOS will need to build and composite views in memory for presentation to the GPU. Depending on how many frame buffers are required the iPhone 6+ could potentially use a lot more memory for a specific application then the iPhone 5S, resulting in memory issues and frequent app shutdowns.
This issue can be seen quite prominently on the iPad where it has to recycle it's memory regularly most likely due to having to clear space for graphics compositing reasons, this was exacerbated by iOS7 and it's use of various shaders to provide blurs. The WWDC 2014 videos covering advanced UI effects explain how they make use of multiple frame buffers at different resolutions to achieve these effects, each frame buffer will use up a chunk of this memory.
The iPhone 6+ will be rending at 1242x2408 which is 4.11x the number of pixels of the iPhone 5S and hence will use 4.11x as much memory for compositing. In addition the iPhone 6+ then has to then down sample this render for the 1920x1080 screen, ultimately the memory used to display the screen also has to live in memory, resulting in an even higher use.
The iPhone 5S feels much faster then the iPad Air/Retina Mini and it's always been my feelings this was a combination of graphics chip fill rate and memory limitations. Unless the memory and GPU are beefed up in the iPhone 6+ this could be very much like the 3 duds Apple have released before:
The iPhone 4 which suffered lack of memory and poor graphics performance for the resolution (it sold well so will always be remembered for this instead of it being fundamentally a 3GS with a higher resolution)
The iPad 1 which was essentially an iPhone 4 with an even higher resolution screen but half the memory (it was really bad)
The iPad 3 which had woeful GPU performance for the resolution (replaced after 6 months it was that bad)
