I am sure it's going to depend on your combo so there really is no correct answer.
I've seen Vigilantes claims, like an ad in a magazine that states their converter will reduce ET .7, which I think was on a 3800 SC Car. Either their converter is really good, or the stock one was really bad, all relative I guess.
Will a Pat's converter shave almost .7 off a stock GN? Probably not but it will be quicker since it's lighter, more efficient and can be locked at WOT more than the stock one can.