I just did a test with a Slot-it GT40. The original Slot-it car weighed 63 grams so I wanted to add 40 grams to bring it to the same weight class as the LMM GT40 which is exactly 100. There isn't all that much space to work with so I just added pieces of lead at convenient spots, and stuck them in place with hot glue for a quick test .
Two bars went inside each of the side panels and then a rectangular sheet upright on the back of the interior tray, 25 grams in all.

Then I put two more pieces of lead sheet in the chassis, 15 grams in all, inside and in front of the motor pod.

The car runs very nicely, very similar to the LMM version. The chassis and motor have no problems hauling the extra weight and it handles really well. The top speed is very high but the added inertia makes the car react slower and
that is very enjoyable. I guess the thin shell of a body will brake more easily if it's raced in anger, with that extra weight, and maybe there will be more wear on the drive train too?
In any case this was a fun test and there is room for some more experimenting.