I think you are on to something by adding all that weight. I am running mine with a saddle pack conversion but have recently added about 4 oz of weight to the truck to start. I have a theory about why this truck is so loose compared to the Losi which is just dead stable.

I believe the truck is just too light. This would not be a problem if the bodies were lightweight, but unfortunately they are not. Therefore, the relative CG of the truck is much higher than the heavier trucks with a body installed. Due to the lightweight of the truck, the truck has to be sprung softly to generate grip, and this makes the truck dump the suspension in hard corners forcing people to "drive it like a 2wd". If the truck is sprung harder, then it just lacks the necessary traction overall.

I will experiment this weekend with a heavier truck and will add more weight and spring the truck harder until I get a more stable and easy to drive truck. This truck is 4WD and it should not have to be driven like a 2WD.

Personally, I still have not had trouble with the stock slipper, but I am sure that with the heavier vehicle, the clutch basket will be a must, so keep that in mind.