Here’s the one I made.
Machine was built as dual extrusion but removed 2nd nozzle to make a 2-1 Y piece both Heaters run independently so can heat up just one. Machine still thinks it’s dual extrusion. To get to higher filament feeds I need to convert to 3mm and can then probably go bigger than 0.8 but I can say it needs a longer melt zone to keep up.
If you can figure out the energy in watts needed to melt filament per mm you can figure out the power and heater capacity needed to melt the desired Filament (it’s how someone explained it to me, I have no idea how)
The two heaters where stable enough to complete a 54 hour print
A local guy named Hanse focushe built a massive machine he named the cheetah it has a massive nozzle and he prints with abs pellets.
Plastic is an insulator, so heat conduction through the filament to melt the core before it hits the nozzle is the bottleneck in most printers, not heater power. A long hot block like a Volcano (or bigger) is required before you need to worry about heater power at all. (I’ve done the math on melt power, it’s a factor in temp PID stability but not max flow rate in a Volcano.) The biggest factor is residence time in the hot zone.
Even in a Volcano, only 5-10 watts of power is spent melting the filament at max flowrate. The rest is lost to ambient. Heater power is not the issue here, simply surface area of polymer in contact with heat.
@Rene_Jurack not really, but I see some usefulness in big machines. @Ryan_Carlyle sorry, didn’t know, as far as I was actively developing and testing nozzles this was golden rule, I think all the major players still carefully follow this rule. Laminar flow will give you nicer looking prints and as for the backpressure, this is also a feature 
I recently calculated that a flow of 40 cubic mm PETG per second takes round about 10 watts (if there are no other losses).
@raykholo we were extruding fine but PLA was running at 280 to get it to melt right. At about 45mm/sec