IANAE (I am not an engineer), but I've been wondering: wouldn't it make more sense to limit the length of the melt-zone and hot-end as much as possible? I've been having a lot of problems with nylon causing jams inside the heat-block, this being the region where the filament is no longer rigid, but becomes squishy then liquid as it is heated.
In my extruder (E3d v6) there is a cold section, then a heat-break then about 20mm of hot-end.
If the heating of the filament took place at the last possible moment, say the last couple of mm before the orifice, wouldn't this not only alleviate jamming problems caused by soft filament deforming, but also allow for much more precision with extrusion - less hysteresis due to the reduced volume of semi-liquid material.
I'd imagine that this would work by having a heated nozzle tip, say a nichrome ring around the nozzle orifice, and as the filament hits the inside of the bottom of the nozzle it melts and is forced out the hole.