I put together a 17.5 buggy with 16ga wire and didn't see any ill effects or heat in the wiring, so I built my brand new 7.5 (fantom/orion r10) buggy with 16ga as well. All good. I count grams and 12ga wire is heavier than I thought. It may be a small amount of weight that's very low on the chassis, but every bit counts.

I mainly do it because it's so much easier to solder and lay out the wiring. That little bit less material makes it so much easier to run wires through tight spaces and curve around obstructions. 12ga insulation also slips right over 16ga insulation, so I can run all black wiring with little colored markers. I'm not rewiring everything, but I'm also not going back.