The 6502 does something similar with its use of the "zero page", remember back then the disparity between CPU and RAM speeds was alot less than it is today.
Yes but INX is 2 cycles and the zero page versions INC $55 and INC $55,X are 5 and 6 cycles, respectively. These regs are 2.5x to 3x the speed of ext mem. So having no internal regs would just be a disaster.
