It’s a minimum weight and remember we are talking about a fairly high level of racing, not indoor rental karts or club racing. A minimum weight of 345 puts basically any driver who weighs more than 160 at a weight disadvantage, as they won’t be able to get down to minimum weight. No one wants to pay a couple thousand dollars and 4 days to go compete at a high level only to be 15 pounds overweight and at a 0.2-0.3 disadvantage all weekend that you can’t overcome.
The video with Aitken is misleading. A rental kart weights far more than a race kart, so the percentages are different, and the track conditions are vastly different to what you encounter in race karts. The benefit of more weight providing extra traction on a slippery indoor track might be legit. In rain situations, heavier karts sometimes handle better too. But a heavier kart will always be slower in some regard, especially on an outdoor track where it is affecting your braking, acceleration, and cornering ability. And remember that a race kart functions differently than a rental kart. The chassis is flexing and lifting the inside rear wheel, so weight balance becomes for more important.
We’ve had this discussion before:
General consensus is about 0.1 per 10 pounds. That can be a lot when you’re looking at the time charts of a big high-level race. Could be the difference between 1st and 5th in qualifying.