Different lengths of filament is extruded with different temperatures
Obviously, your extrusion process is troubled by a lot of pressure. This can be seen from the extensive experiment you conducted with PLA extrusion at different temperatures. Please do note that 230 &deg;C is considered pretty high for PLA! Usually it should be in the range of 185 - 205 &deg;C:


  In general, PLA filament settings have an optimal printing PLA
  temperature range from about 185C to about 205C. If you’re using
  1.75mm as opposed to thicker 2.85mm (or 3.00mm) your optimal print will be closer to the lower end of this PLA filament temperature
  range.


The temperature dependency of filament diameter is explained that small diameter filament warms up way faster in the heating zone of the hotend than large diameter filament as the heat travels less far to the filament core. Basically, with 1.75 mm filament you should be able to print at 195 &deg;C. The pressure that the filament exerts on the hotend and extruder is clearly too much and leads to skipped steps.

It is strongly discouraged to create a function of steps per millimeter (or an over-extrusion by specifying a more than 100% flow modifier). This is a mechanical issue that needs to be fixed by addressing the hardware problem. Usually this is done by:


fixing the extruder; 


is it skipping steps? 
does the filament tension get too high that it skips back pass the extruder gear?
does increase the stepper current work?

fixing the Bowden tube;


is it clean?
is there too much friction?
are there kinks?

fixing the hotend;


is the temperature that is reported the correct one? (thermistor problem?)
is the heat conducting properly to the nozzle?
is there a tolerance issue in the hotend/heatbreak?
is the coldend properly cooled?



A few of these you already explored, others you have not. Unfortunately, you have to do a little more troubleshooting the get to the bottom of the actual problem that is causing this pressure preventing the extruder to extrude the proper commanded length.