Hey Blindside!
Yes, unfortunately it really is that big of a difference. The 9hp vs 6hp weight penalty does change marginally, but there is still a huge difference in power to weight ratio between the weights, even though the power has gone up "50%". A key note is to remember that acceleration is not the only factor in performance/lap time. The cornering speeds at "traction limit" corners go down the heavier you are, because the cart is still running the same width/diameter tires for the heavy guy and the light guy. This is especially apparent in the low/medium speed hairpins where the limit of traction of the tires is easily reached. So, not only is the heavy guy getting killed in power to weight ratio, but TIRE to weight ratio. Also, braking zones are increased for the heavy guys and due to the peaky nature of go kart engines, heavy guys will tend to work with a lower power band of the engine! Thinking about all of this, the heavier guys are literally slower on every part of the race track, at every moment, simply because of the increased mass/inertia/momentum/kinetic energy they are forced to deal with. In an
extreme comparison, this gives a bit of insight as to why F1 drivers are so keen to shed 5-10lbs of weight in the offseason, or why a team obsesses literally over OUNCES of weight on their cars, and will spend many thousands to do it. And they don't even reap the full benefit of "losing the weight" since the cars are spec-weight, this is only to have more "available ballast" to be able to place the weight in the car where they see fit to achieve an optimal weight distribution. This should give people a sense of just how important weight is in motorsport. And don't even get me started on downforce.
Going off on another tangent (for the other folks newer to "racing physics"),
There is one form of motorsport where weight may help increase performance: drag racing. And this applies specifically to putting more weight over the drive wheels so that you have more net traction out of the hole. That's where this applies to full benefit:
Fr = μ x N
* Fr is the resistive force of friction (how much traction/grip you can achieve)
* μ is the coefficient of friction for the two surfaces (Tire vs Road)
* N is the normal or perpendicular force pushing the two objects together (Weight of the car, specifically weight of the car on the drive wheels)
Following along with this, I also find it a funny misconception by some noobs I have met at the track that think more weight = more traction in corners. LOL. That would be true if you didn't take in to account the lateral loading on the car/tires from centripetal force, a force that wants to pull the car outward, in a perpendicular direction opposite of the direction of the corner.
(Think about a ball on a string: swing the ball around and the ball wants to fly off of the string.) The more the ball weighs, the greater the force (Force = Mass*Acceleration) of the ball wanting to fly off of the string. This compares to the increased weight of the driver. Also, the faster you swing the ball around, the greater the force. This compares to the cornering speed of the cart/driver. At some swing speed, and some weight of the ball, that string is going to break. Think about that as the limit of traction of the tires. Putting it all together, something has to give. In order to get the same cornering speed, either increase the tire, or decrease the weight of the driver. When you can not do that, the only thing left to do is slow down for the corner.👍