I have the PowerFlex one I got from TTstuff and I only have a tiny tiny vibration around 1K rpm, I actually don't notice it anymore... My downpipe no longer hits and the gas pedal also feels better when I first press it.

 

do you find the gas pedal to be a little stiffer by any chance... the minute (cincy this is what i meant by negligable) vibrations i feel them at the gas pedal just as im reachign 1kish rpm... and also at the rear pillar... well the sub definetly has loosened the little plastioc clips that hold the plastic moldings to the trunk hatch... but thats another story... or rather project on my to fix list lol

 

my steering wheel is the only thing in my car that really vibrates and thats barely noticble now.