Paced Auditory/Visual Serial Exponentiation Test
Since someone suggested a multiplicative version, I thought the obvious development of this idea would be exponentiation. Perhaps someone somewhere will develop it (and take the credit for it). I recommend the digits be restricted to 0, 1, 2, and 3, just for the sake of not making it strictly an issue of calculating large numbers - but rather, and here's the important idea, doing a maximum number of operations in one's head quickly. I'm sure it will strain WM a lot more as well.