I wouldn't worry too much. Definitely change your fluids more often than you otherwise would but I'd say once per season is sufficient. You also might want to consider switching to synthetic trans. fluid. I use Motul synthetic in my trans and regular Subaru fluid in my rear diff and it works perfectly.
A good way to understand oils, is take an organic chem class. Oil is organic chemical bond. Synthetic how ever isnt all organic. If you took this class you could find out what happens when an oil bond is heated. Also how pressure effects oil bonds. Also with a physic call you could find out Mu to certain oil weights and what happens when oil bonds fail. My advise; it is really hard to say that after a hard day of racing it has caused these bonds to fail. You need measurement to understand if you have broken down your oil. Now if your oil failed durning the race you have already caused damage and you might not even know it. Changing diff oil after every race would just ensure that you have good oil. You might not have even gotten close to hurting the oil in your gear box. So... with this being said...go understand what oil is... down to its bonds, measure it, understand it, and know what its limits are. I know we all dont have time to take these classes, but there are books that can help us. This would almost grantee that your oil would be in working order for every race. I dont know what to say to you. I dont know how hot your gear box gets. I dont know what weight of oil your using. I dont know what chemical make up your oil is. I would call the oil company that makes your oil and get info on it. What is the highest temp before failure? What its shelf life? How does it respond to pressure? With measurement we can prevent.... ALSO we can UNDERSTAND. This isnt a rant or a flaming. I want to help you, because the more we all know that more we can help out ourselves