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They are very much related. I wanted to achieve a strong and fast print with a 0.4mm nozzle, as well as trick the printer into behaving itself on the bottom layers.

I rely heavily on increased flow-rates in spiralize mode (vase mode) to achieve the first goal. You are correct to assume that I am using the higher flow-rate to achieve thicker walls without swapping the nozzles out. I do not have Simplify3D at my disposal, but I would guess that the extrusion multiplier is the equivalent to Cura's flow-rate adjustments.

In Cura, there are only two flow-rate settings available. You have initial layer flow-rate and flow-rate for the remainder of the print. The flow-rate for the remainder will be used on my second and third bottom layers. With the extrusion width set to 0.4mm (default setting for Cura's Prusa profile), the over-flow on the bottom layers causes the printed lines to overlap and interfere with one another, resulting in a literal hot mess. Setting the line width to 0.6mm (to your point, 0.56mm would be the theoretical value), results in a nice clean bottom.. which I think you'll agree is desirable.

In short I'm just too lazy to swap nozzles, but I want nice thick spiralized parts. The side benefit to using this method over using the 'correct' nozzle diameter is that - for a given wall thickness - you can get very strong, very high resolution parts by over extruding. Brook Drumm (Printrbot) used this to great effect to create prosthesis printers for some of his clients. He leveraged lotsa extruder power and up to 300% flow-rates.

I can't say I've run a comparison between my 'method' and a part printed with a 0.6mm nozzle at default settings, but it would be interesting.