So it's a software issue? Because those aforementioned devices never got smoothened out, even after software updates.
Could be at different levels in the pipeline, without knowing what HiDPI entails (converting from a 1:1 point/pixel to a 1:4 point/pixel system) as far as algorithms goes and what impacts it has had on Apple's Quartz model, we can only speculate. Something that is sure, it's not the actual GPU itself, since those have been capable of these pixel fill rates from a while, and obviously, 30" ACDs were running off these IGPs 4 years ago with the introduction of the mini Display Port, finally enabling higher than 1920x1200 resolution for external monitors.