> Honestly haven't noticed a single thing.
I've got a coworker sitting right next to me with a HiDPI main + non-HiDPI secondary screen. X or Wayland, it's a thousand cuts of hell: set native resolution and render scaling to 200% and you get some (most) apps at 200% on the non-HiDPI screen. Drag an app from HiDPI to non-HiDPI and notice how the overflow is wrongly scaled. There's no way to render at 200% on a non-native resolution then downsample to native, you have to set a non-native resolution, upsampled by the GPU/panel, which results in a useless blurry mess.
> it doesn't get burning hot to the touch when doing actual work on it. Nor does it sound like it's about to take off.
Early '13 and mid '14 13" rMBP here, VMs don't lag, slightly warm, but in no way "burning hot". Early '13 is dead silent thanks to dual fan design, mid '14 slightly less so, and only when I peg all cores for long enough.