Good points - esp about how we've got essentially limitless motorage these days!
Yeah, I thought about that grip issue with the coefficient of friction etc, but if you think about it, it's not a big advantage at all unless that relationship is actually "better than linear". Because the force of friction = mu * m * g you'd have proportionally more frictional force with more mass, yes. But on the other hand you'd also need just as much more force to accelerate the car in any direction, since f = m * a. Hmm. I think I'll also go into ponder-mode...
As you point out it's also interesting to remember that when we say the cars are 1:10 - we're really only taking about length - one dimension. A 1:1 car will cast a shadow 100x bigger - two dimensions. And have a volume 1000x greater - the third dimension. So if that 1:1 car is made made of all the same materials, it'll have 1000x more mass!