Part of the extended instruction mode is a change to the data addressing behavior when a=0 and f<=0x5f.
With extended mode on, a=0, f<=0x5f map to memory location FSR2+f instead of 0+f. Behavior for a=0 f>=0x60 is the same as without extended mode (map to 0xf00+f).
I suspect this difference is very important for running C code compiled for the extended instruction set (to support local variables).
For details, see the "INDEXED ADDRESSING WITH LITERAL OFFSET" section in the datasheets.
It's on page 92 in this PDF: http://ww1.microchip.com/downloads/en/DeviceDoc/41412D.pdf