I'm utterly torn with which motorsport lens to buy, I can't decide if the 70-300mm G SSM will be fine (compact enough to go travelling with me as well) or to get the 80-400mm G SSM (too big for anything else, but a complete weapon). Leaning towards the 70-300 as I can always use the Smart Teleconvertor function for extra reach.
The Sigma 100-300 f4 if you can find one on the used market.