I've been using the attached 8MHz version of Optiboot. I modified the boards.txt entry for the 8MHz Pro Mini as follows. Also see similar thread here.
pro328.name=Arduino Pro or Pro Mini (3.3V, 8 MHz) w/ ATmega328
pro328.upload.protocol=stk500
pro328.upload.maximum_size=32256
pro328.upload.speed=57600
pro328.bootloader.low_fuses=0xFF
pro328.bootloader.high_fuses=0xDE
pro328.bootloader.extended_fuses=0x05
pro328.bootloader.path=optiboot
pro328.bootloader.file=optiboot_atmega328_pro_8MHz.hex
#pro328.bootloader.path=atmega
#pro328.bootloader.file=ATmegaBOOT_168_atmega328_pro_8MHz.hex
pro328.bootloader.unlock_bits=0x3F
pro328.bootloader.lock_bits=0x0F
pro328.build.mcu=atmega328p
pro328.build.f_cpu=8000000L
pro328.build.core=arduino
optiboot_atmega328_pro_8MHz.hex (1.43 KB)