panty vac

I've run one on my street car for years. Here's the jest of it. They work best with open headers.... They do work with an exhaust system but efficiency (and check valve life) are reduced the more restrictive the system gets. The more "free flowing" your system is the better they will work - and last. Run straight through mufflers - nothing baffled, and keep your tubing simple and as short as possible.
I run DynoMax Ultra Flos on my car with 3" tubes and while I've never measured it I have significant vacuum at the breather just at idle. The more back pressure your system has the less vacuum you will get - and the faster the check valves will deteriorate. I suggest checking the function of your check valves periodically by pulling the hose to the breather (or pulling the breather itself out of the valve cover) and putting you hand over the end with the car running. You should feel a steady vacuum. If you feel any pulsing or pressure the valves are shot and need replacing.