We're going through an exercise to rigorously test the effects that caching has on non cached performance. The logic is simply that the more you offload to cache, the more system resources are available for running PHP and Magento code.
With that said, so far, we are going to use shopping cart/check out as our main non-cached process. I thought it might be beneficial to ask what else you'd want to see as part of the test harness? Our goal is to publish something that anyone can download all the assets and replay against their Magento store to help tune performance.