Yes of course weight makes a difference.

Alluminium and any other metal WILL weigh the truck down and therefore make it slower. If you read the race forums you will see that everybody who races anything, wants it to be lighter.

Heavier trucks CAN be more robust, but, not neccessarily.

The only parts I would have metal are things like gearbox housings and bulkheads to remove the 'play' from the driveline.