By rendering to a larger buffer they can ensure that any raster assets may use an integer scale factor between devices. If they rendered to the panel's native resolution (1080p) they'd probably have to use a 2.5x scale factor, which is not desirable.
The output will be slightly worse, but at 400 PPI you probably won't be able to tell without a magnifying glass. The current Retina MBPs do this in their higher res modes and it's pretty hard to notice.
On battery life: perhaps the tradeoff of using a 1080 panel outweighs the scaling.
Also it leaves the door open to switch to a full resolution native panel in an iPhone 7+ with no hit to existing apps or developers.
Huh, I don't know that much about GPU's, but surely they can render fractional pixels directly without downscaling shenanigans. Which actually means that GPU will convert full pixel values to fractional before rendering. Rendering to a larger buffer and then downscaling is actually a form of antialiasing, albeit an inefficient one (if I'm not wrong). Also, rendering without antialias would result in rounded full pixel values. Obviously, bitmaps will be downscaled on the GPU using whatever filters are set for them (just like before, if bitmap is larger than the target size).
It's not so much about the GPU rendering fractional pixels, it's that it makes it much trickier to create neat assets that work at non-integer scale factors. Generally you want to ensure your raster assets have details that lie on integer boundaries at all scale factors, throwing in a non-integer scale factor as a target makes it more difficult to do that.
The output will be slightly worse, but at 400 PPI you probably won't be able to tell without a magnifying glass. The current Retina MBPs do this in their higher res modes and it's pretty hard to notice.
On battery life: perhaps the tradeoff of using a 1080 panel outweighs the scaling.
Also it leaves the door open to switch to a full resolution native panel in an iPhone 7+ with no hit to existing apps or developers.