Mirai is one of the first production HFC vehicles, well below what's feasible. Total solar energy is not really a constraint, refueling time and amount of stored energy are.
Total solar energy is a constraint, as someone has to pay for the cells and to have space to set them up. If you want to power your own car, don't you think it makes a difference whether you need 1x or 3x the area covered with solar cells?