my 2 cents:
1. the weight added is certain to slow you down, or perhaps cause imbalance in the car. the heat lost due to "increased thermal mass" is insignificant, meaning that the efficency gained by the heat sink can be EASILY compensated for through car setup, driver's conditions, road conditions, and basically any combination of parameters.. (i have no numbers to back this up, but neither have i seen any numbers posted on these "wire heat sinks" to prove that they do anything.)
2. If the motor is heating up to the point where it is unsoldering wires, then maybe the wire joints are cold, the car is geared improperly, or the drive train is inefficient. I am sure there are reasons why this is happening. Either way, if the motor is running that hot, I suggest getting a better motor heatsink/fan combination, rather than a "wire heat sink". For sure it will be more effective.
basically, I think money can be better spent elsewhere.