I think really you want a "quality" option per-layer so you can render at a (custom) smaller size and stretch up to fill the display, similar to how low-quality fullscreen scaling works.
I'm afraid we will not be adding any significant new features to C2 from now on, and this kind of improvement is something we'd only consider for the new C3 runtime.
Also for what it's worth, this is one of those surprisingly tricky optimisation areas: using a lower-quality for a layer will cause it to use its own texture, which in some cases can use more fill rate than if you didn't lower the quality at all. The overhead of the own-texture copy has to be exceeded by the amount of GPU time saved by the lower quality rendering. This is not a simple equation and in some cases will just make things worse.