A web page is not scaled down more, it's the physical width of the display no matter what resolution the display is. A typical website is designed to work at 960 pixels wide (1024 - scroll bar). If this is displayed on a 4.3" qHD display, it will be 540 pixels wide, on a WVGA display it will be 480 pixels wide - but still the physical width of the display. The higher resolution makes it more readable, although present qHD displays use pentile arrays which isn't great.
Acknowledged. you're right on this point. including your observation about PenTile
I believe "retina", as Apple call, means individual pixels can't be seen and that has been shown to be 300 PPI at usual phone screen viewing distance. It varies from person to person and age decreases the PPI you can determine.
"retina" by definition means a certain pixel density and viewing distance where your eyes can no longer resolve the shapes of individual pixels. For a 3.5" screen, Apple defined retina to be 300ppi when viewed at around 12inches.
But take a 21" desktop monitor. When viewed at around 28" (a reasonable distance), it becomes a retina display as well even though the pixel density is only around 120ppi. Same applies to 50" HDTV. It becomes a retina display at around 6ft, a comfortable viewing distance, but the PPI is less than 100ppi. That's why I said that "retina" is a function of two parameters, not just ppi.
Since many Android phones have much larger screens than 3.5," a 300ppi display would have a very high resolution. Performance will definitely come to play, and given that framerates on Androids in single-core phones have sorely underperformed compared to iOS devices (if you don't believe me, do a pinch-to-zoom side by side), having such a huge resolution to render is going to keep things choppy. And personally, I'd rather have smooth scroll/zoom than ultra-high resolution.
TBH, when I hold my 4.3" WVGA at 12" away I can't see individual pixels. I can at around 8", but at that point, I'm straining my eyes a bit. I think qHD is plenty awesome for phones. Anything above that is just bragging rights, rather than meaningful, observable difference.