It's gotta be integrated together (and possibly be at a smaller process size) before you get all the power savings. If you have a real 6502, it has no internal RAM or cache, and you need to send power back and forth to the RAM chips. (I presume it's SRAM, but it could also be PSRAM)

If you ran a 6502 emulator on a cheap ARM SOC, it would be much lower power.