@Danal I recall a discussion involving @deckingman some time ago which touched on the elasticity of the 'shot' of melted (hopefully) material in the hotend, and the effect of this on retraction requirements. There was also theorising about the effect of extrusion rate on the average viscisity of hotend melt, down in the vicinity of the nozzle. The thread was contentious, and I remember thinking that one reason for this was the existence of many variables and influences which were not acknowledged fully. I see this in Moldflow analysis of mould cavities which I occasionally do in my day job, where resin melt heating occurs in regions of high shearing rate (e.g. constrictions), even a long way from the gates where the flow front has cooled.
So, for sure, knowledge of average pressure and melt temperature determines a lot about the rheology of the melted material being extruded from the nozzle, and thus instantaneous extrusion rate. This should help with improving extrusion reliability.