I have one on my car but usually keep it closed because I have an ATR exhaust, which I feel it flows fairly well and I kinda like being quiet.
That said my best time through the exhaust is a 10.75@124.86 and best MPH is 10.76@124.95, on my last time out for 2003 I decided to open the cutout and see if it'll help and it went 10.73@125.18 and 10.69@125.80 on my first two passes.
So in my case it did seem to pick up some.
HTH,
Steve