I've put weight on the insides of wheels to balance them.. but never for "more grip".
Seems like there are much better things to do than to add to the rotating mass of the drive train.
There are an infinite amount of adjustments you can do to your suspension not to mention different tires and compounds.
The front tire grip can play a role in your rear grip as well... if your front grip is too great, the rear can break loose especially in the corners. Again, your front suspension can be adjusted as well.
Don't forget about adjustments to your slipper clutch and differentials.
You can change to different wings on a buggy and bodies on trucks. They are NOT all equal!
Adding weight to the wheels just sounds like a bad idea. In all sorts of racing, not just r/c, the idea is to go as light as you can for speed.
Just my opinion..