since this topic popped up in a google search for me.
I thought it would be helpful to add:
from the data sheet: 9.2 BOD Disable (p.40)
"When the BOD has been disabled, the wake-up time from sleep mode will be approximately 60
µs to ensure that the BOD is working correctly before the MCU continues executing code."
so... wake-up time from the deepest sleep mode (Power Down), is going to be less than 1 uS, regardless of the ATMEG328P's start-up time fuse settings.
However:, disabling BOD in software (just before sleep) is going to add 60 uS to your wake-up time.
The irony is that disabling BOD in software (or even fuse settings) allows for significant power savings during sleep: (18.3 uA w/ BOD vs 0.1 uA w/o BOD) *according to my at-home tests w/ bare 328P-pu running internal 8mHz.
So it seems if you are looking for the best possible power-savings in sleep mode AND the fastest possible wake-up times... then your only option would be to permanently disable BOD (by changing the fuses).