You still have to run romex for the other circuits. But it's not hard to run those on a separate breaker. Rooms often have like one and a half 15A circuits each because lighting used to draw so much power but now you can drop it down to one 15A circuit per room and run a single circuit that switches out to the entire house off of one 15A breaker.
> what's the difference between a power supply running on 48 volts DC versus 110 or 220 volts AC?
It's already DC. A properly sized buck converter based LED driver is going to have like 80-95% efficiency when the LED is on (and it's trivial to draw no power when off) and they are like 0.25USD per IC.
They are going to have equal or better performance than AC->DC switching mode power supplies because buck converters are already the simplest and most efficient type of SMPS.