Well No, It's not just esram that means a lack of 1080p, it is also the power of the consoles GPUs, but this is a significant factor and would help determine where a developer would cut scale back, regarding fitting it into 10mb if we look back at my previous post ad just throw 720p numbers into it we can get:
Back Buffer:
1280x720 [Resolution] * 32 [Bits Per Pixel] * 2[FSAA Depth]
= 58982400 bits = 7MB
Depth Buffer:
1280x720 [Resolution] * 32 [24Bit Z, 8Bit Stencil] * 2 [FSAA Depth]
= 58982400 bits = 7MB
Front Buffer:
1280x720 [Resolution] * 32 [24Bit Z, 8Bit Stencil]
= 29491200 bits = 3.5MB
For a total of 17.5MB with 2x AA, of course this doesn't fit in 10mb edram, however if we look at the memory setup of the xbox 360 bandwidth we get this:
That has bandwidth of just 32GB/s for Esram compared to the 22.4GB/s to main ram, I comparison to the PS3, it has similar bandwidth between GPU and main memory, transferring data from esram to main memory and back on the xb1 is relatively a much bigger hit to performance than it was in the 360, All three of the buffers at 720p and 2x FSAA fit into edram and can be moved between edram and main memory with less of a performance hit than if the same thing was done on XB1. For instance on XB1 it would drop a potential bandwidth from 140gb/s down to 68gb/s for a drop of over 50% (and around 60% slower than PS4), compared to the drop in the 360 which would only be from 32 to 22, which is around a 30% drop (and around the same as PS3)
Also further in that thread it is clarified by Kagari that a Japaese developer was having similar Esram issues.
We don't know for sure what it the main cause of sub 1080p But I would assume both the lower Graphical power and the esram will both contribute.
We are in the same situation for tiled deferred rendering as well AFAIK
I don't know, I heard that each can display four rectangle Meaning that 3 of the rectangles could be flagged as empty and only display one portion of the screen in the buffer. If that was the case they could Display 1 quarter of the screen at 1080p (main character) and another at 3 quarters of the screen at 720, I think this would save memory, However it is far more likely the additional display plane would be used for a 1080p UI stuck on top of a 720p screen. But I really don't know.