http://www.audiworld.com/model/s5/content.shtml says the S5 does indeed come with a 60/40 split folding rear back. Failing getting an answer to your question, the options are:
Option 1 - get his dear friend sakimano to take some wheels
Option 2 - delete this thread, start a new one asking if Axel wants to do some pulls against an S5 and offer Axel $$$ if he can win S5 vs SC’d S4 and his S4 has 4 winter wheels in it.
In all seriousness, I did help out an A5 owner once by picking up some wheels, we didn’t even consider stuffing them into the A5. He paid the small price of hearing on a regular basis how much faster and more useful my car is.