How were they designed, to do what purpose? If it wasn't to eliminate the drone - they do, if it wasn't to eliminate the cats - they do whether you have stock exhaust or after market. If these weren't the intent of the build then what was?
I have them with my cobra 626 exhaust. IMO, I think they were designed to add a slight backpressure to some of the aftermarket exhaust systems, but the stock exhaust already has restrictions so if you add loopz, it might have negative results.
2013 F6B 1800B Don't be a thermometer, be a thermostat