Coming from a testing and performance measurement background, I can testify to the importance (and difficulty) of assessing the performance of a Sharepoint implementation prior to deployment. James Edelen has a great post on stress testing using Microsoft Application Center Test. Read it here.
While this doesn't address functionality or user-experience issues, it will give you a working data set for determining if your configuration is adequate for anticipated usage patterns.
TIP: Run your test case(s) iteratively after the basic install (to establish a baseline), then each time you make customizations to templates/site definitions, add lists and content, import documents, created custom web parts, etc. This way you'll know where your tipping point is and which features consume the most resources.
