I think this would make a cool project to do just from the proof of concept, one of the main hurdles would be to have the liquid tube temperature controlled and insulated enough that there would not be any fluctuations in temp because at any length even the slightest temp related expansion is going to show up in the output 
In any case...
Alright see if you can follow this. Melted plastic. Extruder is attached to stepper motor that pours into a reservoir where it is further heated. Send the melted plastic along a heated pressurized tube all the way to the secondary extruder which is cooled either using coolant or a liquid cooling system block. Therefore making it solid again for extrusion.
The pressurized tube and reservoir help push the more solid material out of the cooling extruder.
Ok heres where speed comes in. First off its going to be fast, secondly since its in liquid form it won't have compression.
Compressed gas (or gas-charged) closed accumulator
Piston Accumulator
A compressed gas accumulator consists of a cylinder with two chambers that are separated by an elastic diaphragm, a totally enclosed bladder, or a floating piston. One chamber contains hydraulic fluid and is connected to the hydraulic line. The other chamber contains an inert gas under pressure (typically nitrogen) that provides the compressive force on the hydraulic fluid. Inert gas is used because oxygen and oil can form an explosive mixture when combined under high pressure. As the volume of the compressed gas changes, the pressure of the gas (and the pressure on the fluid) changes inversely. Existing hydraulic accumulator designs are large and heavy due to the need for two storage tanks and do not have the high energy density needed for many applications.It is possible to increase the gas volume of the accumulator by coupling a gas bottle to the gas side of the accumulator. This is mainly done since a gas bottle normally is cheaper to produce than an accumulator.
The compressed gas accumulator was invented by Jean Mercier,[2] for use in variable pitch propellers.
Was reading this from wikipedia.
Kind of the idea. But the reservoir pressure can be raised or lowered using 2 air bladders or liquid bladders that can increase pressure or reduce pressure using hydraulic power (different extruding widths). Plus a secondary inert gas or liquid bladder lightweight liquid possibly…. runs along the hose as well to the cooling extruder which is weird in itself to say.
It can do a burst in quick succession for quick parts and speed. With a dual extruder setup even plastic painting? Eh afterthought, probably not a good idea.
Is the work worth the gain though? Probably not, but is it even possible? Part of me want to say its too complicated and probably expensive to be viable for any current 3d printers or even as a DIY option.
Nevermind just the ramblings of a madman currently far to deep in thought working with his first 3d printer. XD
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