even the ipad uses sub-pixel rendering for its fonts
No, it doesn't. No iOS devices use sub-pixel rendering, ever.
Unless you mistake it for normal anti-aliasing, which is done on pixel level.
I have less than perfect eyesight. I hold my iPad about 16" away normally. No, I cannot distinguish pixels on the Retina iPad at all. Practically I don't care what is true "retina". For the majority of people, Apple's definition of "retina" is good enough.
You'll need closer to 600dpi in order to not be able to distinguish pixels.
If you still think your ipad/iphone is true retina. Put a single strand of hair on the screen, you'll now be able to see the pixels.