I've had my 70 duro installed for about a month now and don't even notice the vibrations anymore. No telling if it's just that I've gotten used to it or if it's gone away entirely. The car does seem to shift smoother now, but again, I might have just gotten used to the feel. I also used to get a rediculously loud banging sound during the 2-3 shift when really giving the car hell, and haven't had it since installing the rear mm...but again, I might have subconsciously knew what I was doing to cause the sound and adjusted my driving style to suit.
All in all, once the vibration becomes a non-issue to you anyomore (which it will given time), you'll hardly consider $80 to be an expense worth losing sleep over.