A floor-mounted pedal box is more advantageous for drivers that ‘heel-toe’ shift with the sides of their foot, rather than actually using a heel and a toe, as there’s more room for movement up top.
With racing pedals for cars, whether you’d prefer floor-mount or top-mount pedals is down to personal preference. However, many drivers find that the movement of floor-mounted pedals follow the natural movement of your foot when seated more closely. To get leverage on the pedal with the ball of your foot may be easier with floor mounted pedals, and with this setup you can free up space above your feet, which is prime real estate.