Extrusion tests, steps per mm. My viewpoint  

Extrusion tests, steps per mm. My viewpoint

I’ve had my MK2S since around May last year. In that time, I’ve made a variety of changes, many of which were inspired by threads here.
Among those changes have been setting the steps per mm in the firmware so that telling the extruder to extrude 100mm actually extrudes 100mm of filament.
My results from doing this were very similar to what other’s have reported, that the factory setting of 161.3 is about 7% too low. I ended up changing it to 171 which caused my 100mm command to extrude 100mm of filament.
I recently was printing some items that require a precision fit and found that they were too large and this appears to be related to the extrusion setting. I made changes to the extrusion multiplier in Slic3r and was able to improve the parts by lowering the multiplier to ~.94. Since this was approximately the same amount I had upped the multiplier in the firmware, I decided that rather than reset the multiplier for every filament, I’d set the firmware back to 161.3.
With the firmware at default, and multiplier set to 1.0, my prints are much more accurate across multiple filaments (PLA and PETG).
So, some questions and concerns.
1) Why does the firmware have the 161,3 setting embedded when it is always (by measuring the filament extruded) wrong?
2) All the gcode on the SD supplied by Prusa prints great with the 161.3 setting. If it needs something else, why do the supplied .GCODE files print so well?
3) Have others had this same experience? That changing steps/mm hurts the accuracy?

Based upon my experience, I think it’s a mistake to change the steps/mm in firmware. Everything is more accurate, at least when slicing with Slic3r, with steps/mm set to the factory defaults. If there are changes needed, they should be made for the specific filament in the slicer.
For testing extrusion, I was using an XYZ calibration cube and an object named Extrusion_Test.stl that I downloaded from somewhere. I prefer that latter as it provides and easy to evaluate fit test. By printing two, you can see how they fit together which gives you an accurate evaluation of the effect of extrusion changes. I've attached the stl.