weight will affect the acceleration of the car but it has very minimal effect on the top speed.
The aerodynamic drag, friction, power loss thru the drivetrain are three of the biggest factors that affect the top speed (assuming power is constant).
So assuming your TC is a 4WD, and the TC shell usually has a much higher aerodynamic drag than the 12th scale cars.. so the top speed of the TC will be lower if both car has the same motor, same battery and same rollout.