There are two places you should focus.
First, in your test environment, you should be using a code profiling tool (like the kind built into VS) to see what code paths are actually being executed. This reveals by far the most details into your app, but it can be difficult and time consuming to set up a good simulation of your production environment and load.
Second, use custom performance counters in your production code. We create our own performance counters, and install them with a setup utility. This adds some complexity, but you really can't beat having them for insight into what your web application is doing.
The act of updating a performance counter is minimal. It's the reading of the performance monitor which can add some overhead (counter-intuitive, I know).
The version of the Enterprise Library Library we're using (3.1) does not have any direct support for custom performance counters, although it does install a number of counters for itself. I don't think these are the ones you want.