Ill take a shot at answering your questions.
First off on most HX boiler type machines the boiler is only filled roughly half-way. So in a 4lt boiler youd only have 2lt of water in it. The rest of the space is needed for the steam. As too whats stopping you from only using 1lt, nothing, as long as 1lt fills the machine to the minimum water level. ;D How-ever, before you do that I think you may be under a misconception. The water used to brew the espresso does not come from the boiler in the commercial La Cimbalis. Room/line temp water is injected into the HX unit in a seperate system from the boiler water. The boiler water is there to heat up the HX, assembly which then heats up the brew water, to provide hot water via the hot water tap, and to provide steam for pressure in the boiler and to the wand(s). If the water level in the boiler falls below the minimum allowed bad things happen. :-/ Your heating elements are no longer covered by water and will overheat and burn out.
The La Cimbalis (and virtually every other commercial or even near commercial machine Ive heard of) have an auto-fill boiler. While most of them can be tweaked for different fill levels personally I leave them alone as long as theyre working properely.
As to the economics of heating up 1lt as oppossed to 2lt in the boiler I suspect it is not all that much cheaper, and in fact may actually cost more. The water in the boiler is heated up until it has produced enough steam to bring the boiler to a pressure of 1.2-1.4 atms. It takes 1 calorie of heat to raise 1 gram of water 1 degree C. So to bring that 1lt of water from room temp (say 20C) too 100C would take 80,000 calories. If I recall my chemistry and physics correctly it then takes 80 calories to turn 1 gram of 100C water into steam. Its been enough years I no longer remember how many grams of steam it takes to put a 1lt volumn under 1.2 atms but I suspect that the energy required is pretty close to what it takes to heat that volumn of water from room temp to boiling. All in all the electricity saved would be tiny, and the possible losses (a burned out heating element) so expensive as to not make the risk worth while IMHO. :)
Hope this helps. :)
Java "The only stupid question is the one thats never asked" phile