So, it looks like I've made a daft mistake. I was checking the weight of each machined part and comparing it to the CAD model. All the machined parts are within 1% of what I expected, but the motors were way off - 450g in CAD when they're 1.8kg in reality! I don't know how it happened as they're correct weight in the Deadlock model. Regardless, I'm going to need some drastic weight saving measures to get this back on track. Current favourite is to ditch the kart wheels as they're the next heaviest part after the motors but I really want to keep the aesthetic of big fat wheels.
:-(