Can I lower flash power consumption by pre-flashing all values to high?
The way to save battery when having a SD card in the system is to use a transisor or FET to switch off the DC power supply to the card connector when you are not needing to access the card for data reading or writing.

You want to keep the SD card GND intact with the MCU GND. So make sure to have the power switch turn the SD card VDD on and off. 

Also to prevent excess leakage into the powered down SD card make sure to set all of the I/O pins that connect to the SD card interface to a low output level before turning off the card via the transistor or FET.

Skip any ideas that you dreamed up regarding to save power by changing the content of the flash memory to a certain default data.