Hi, look not sure what you actually mean, so I donít understand what problem you are facing, but if you want to add lead does this mean you want to slow acceleration down or what?? I am not familiar with the GM Speedoís but maybe you could try a higher drive frequency, that should back the punch off a bit and you would get higher top speed as well, and probably better run time from less weight (by not using lead) and better motor life from the higher drive frequency.
If you want to take away momentum to bring the car to a stop quicker instead of decreasing acceleration, you could try increasing drag brakes and reduce dead band, and maybe even try a lighter armature design, such as a DC motor sports UL arm, or maybe a flat wire design or if you use a 5mm web(Arm), you could try a 4mm version.
Mugen MTX & MRX
"He who dies with the most toys wins" - Phil Harris