I would like to explain it to you in terms of scaling. Deku could never use 100% of his power. Deku can control 45% of his power and here he increases his power by using gearshift. So he creates a fake 120%. So 120% is the combination of OFA 45% and gearshift. If Deku could use 100% OFA with gearshift He could use 185% power and 200% power with others. Also, Deku has an attack speed faster than the speed of light. This sound comes later, he told Shigaraki It is the speed of the explosion that comes after touching it. In addition, gearshift allows the user to change the speed of the things it touches in space. Therefore, an object that is already at the speed of sound cannot achieve this.