If I were to do 10 Sonata Tracks races with the Devon Corp. sponsor, the money stacks so I can get all my winnings by talking to the red lady once at the end. But the sponsor’s item rewards do not stack.
You’ll be given item rewards as if you only finished a single race. You have to talk to her after every race to get the item rewards for each race you do. I suggest that the items stack too so you can talk to her once when you are done and she’ll give you all the money and all the balls you earned for all the races combined.