One simple and easy way to run meaningful performance tests is to take a snapshot of your production data (minus any personal/private information of course)
That's the real trick isn't it? You need to sanitize personal data, but maintain similar cardinality on all the indexes.
Using production data sets is very important. It's also really important to understand what your users are doing or will be doing with your application. It doesn't matter if the dataset is realistic if the simulated traffic doesn't represent realistic activity.