No you do not lose any power, it is a TTL logic signal (5VDC) to turn the laser on and off. The laser is powered separately from the 12VDC. All you are doing is keeping the signal at 0V when it is not turned on, to prevent the laser from unexpectedly switching on. Logic is either on or off, so even if the signal were only 4VDC it would be full power.
I tested Grbl without the boot loader and the pulse was eliminated but many people don't have the hardware or skills to do this so it isn't practical.