The angle is wrong and the brake pedal sits a bit higher up/closer to you than the gas pedal. In practice, my left foot tends to continually tap the brake due to this while also tiring my left hip inductor muscle, though I'm sure this would be fixed with practice. If you see a car randomly tapping their brake lights without ever slowing down or holding them on while accelerating, chances are, their left foot is hovering on the brake a little too close. Holding the gas isn't such a big deal because you're already holding it all the time. Oscillating between 35-40% gas isn't as noticeable as 0-5% brake, especially with flashing the lights.

From a safety standpoint, while youd be able to get a ~250ms advantage by having your left foot ready on the brake, you're not going to leave it there. It's probably going to rest on the foot rest like it always does because the floorboard is arranged like that to hold a neutral leg position. While this kills the speed advantage, it also loses another major advantage: positive placement of your feet. When you use your right foot for the gas, you know exactly where the brake pedal is in relation. When you float your right foot against your seat to flex your knee, you lose that positive location. If you have to panic brake, you now only have a pretty good idea where the brake pedal is. It works out 99.999% of the time. When it doesn't, we get videos of "runaway" cars plowing through buildings. It's usually someone mistakenly mashing the gas pedal because they lost their foot location references. So while you could train your left foot, it has to cover twice the distance - more room for error. It's also pretty cramped in there with current designs, so when I've tried exsctly this, I had a tendency for my right foot on the gas to cause interference with the brake pedal being depressed.

There's definitely times in racing where left foot braking is used at times when you need gas and/or brake in rapid, planned succession/concurrence. Yes, there's times for gas and brake together. Most cars are 2 wheel drive, all cars are 4 wheel brake. This means you can alter the balance of the car by applying both pedals. I've driven 40mph go karts with left foot brakes. Even though I lay competitive lap times, I don't think the left brake is a significant contributor. It's just a compact design choice rather than a performance point. The pedal heights are equal though, unlike normal cars. But that's not to say it's a bad design, just that normal cars aren't designed that way, so the benefits are lost, or even become a hindrance. Perhaps the pedal box design is a carryover from when the standard transmission was standard