Have you looked into the physics of how much weight a wheel that small in diameter and light in weight at full RPM would be off by? From years of personal experience, I know a competition go-kart tire which is approximately 11" across, 5" wide and 35" in circumference, rarely takes more than 1/4 ounce to balance. Competition kart tires spin more than 2000 RPM.
I would be more concerned about eccentricity than weight imbalance. Are the outside of the tires circular and concentric with the bolt hole in the center? The car doesn't have a lot of weight to help press the tire down, unlike real cars. Automotive tires are not perfect circles, and neither are their rims. Combined, these factors cause the tires to be egg-shpaed and have a high spot. One of the reasons automotive tires go out of balance after installation is the weight of the car causes the high section of the tire to wear more than the rest of the tire. Thereby removing material (weight) and causing the tire to now be out of balance. The RC car doesn't have enough weight to cause this self-centering effect. Getting the tires concentric with the hub hole would keep the tire in contact with the ground better.