In theory, how much, if any, improvement should I get by removing Optiboot 6.2 and running just the straight sketch without a bootloader?
Very little. Optiboot does the power-on check VERY early, so almost all the startup time is consumed by the hardware reset and oscillator start (controlled by the fuses. The 65ms is the "slow starting crystal oscillator" start time, and you're probably seeing less that that because you've got the fuses set for internal 8MHz clock. IIRC, the startup time is controlled by the WD oscillator; an internal RC "somewhat close to" 128kHz clock.