800px patching regular games?

i think that if it were possible, it would have a huge impact on performance, especially on old 3ds, new 3ds would probably be able to handle it with less of a performance impact, but i dont know if it would even be possible without the title's source code

 

And using 800px mode in emulators? Wouldn't that make things nicer-looking?

DaedalusX64 would greatly benefit from that, as well as other emulators.

 

Nintendo likely used it as a debugging feature to test the 3D screen mode without an actual 3D screen, and as a result, you could see both the left and right sides at once on the top screen.