I don't think anybody is arguing the merits of left foot braking on a full-on gravel spec rally car, with brake discs the size of bin-lids, and fully adjustable bias, and massive power outputs.
However, when on tarmac the level of grip is so much higher and in a car with relatively modest power levels, non adjustable brakes and, dare I say it, perhaps a driver of less experience/ability than the established super stars of the motorsport world, any benefit of this activity would be marginal and driving in a more conventional manner probably just as quick.
The risks of de-stabilising a car by left foot braking on the road I feel would outweigh any possible advantages.
Andrew