First off, you won’t ruin your 20″ wheels in an autocross event, but you will wear your tires at an accelerated rate. One event won’t make much difference; but if you do a whole season, the wear will be noticeable over daily driving, though it vastly depends on the type of tire you’re running (extreme summer performance, max summer performance, all season, etc.) as to how much.
Now, to answer your question, I would stick with the same size on all four corners (choice C). But if you need to run mixed sizes, I would go with taller in the back, shorter in the front (choice A). Those sizes mean roughly a 28″ tall tire in the back and 26.3″ in the front. I’d never do that on my AWD (or any AWD for that matter), but your RWD should be okay.
BTW, those are pretty narrow tires for 340+ HP, so you’ll probably be slipping and sliding out there. 😉