Prusa MK3S 0.8 mm nozzle layers separating
After printing ~30 calibration test cubes I finally figured out the perfect settings. 

Apparently, the extrusion width in my slicer was set to a certain number, instead of 
percentage. I was printing at 0.45&nbsp;mm extrusion width, which was ~105&nbsp;% of the 0.4&nbsp;mm nozzle diameter, but less than 50&nbsp;% of the 0.8&nbsp;mm one. Changing the value to 130-140&nbsp;% 
solved the majority of my printing problems.

As I predicted, the 230-280&nbsp;℃ nozzle temp. was ridiculously high, even with the 
0.8&nbsp;mm nozzle.  I gradually lowered it to 210&nbsp;℃ (ironically, the value I chose at the very beginning). 
 There was some warping, so I cleaned the heatbed from the glue, put the glue again and decreased bed temp. to 50&nbsp;℃
 Finally, I decreased the flow from 1.3 to 1.05 and everything seems perfect now.