JPop

With actually owning an Airaid TBS, a freebie when I bought my Airaid intake at Summit Truckfest, I can assure you that the differences using the spacer actually come at low rpm. I have taken it on and off a few times (currently it's just sitting on my toolbox) and the only notable difference is in low end throttle response. No measurable increases in efficiency, no noticeable power increase and just a little throttle response gain that is used up before 1500 rpm. Certainly not the kind of thing to throw some coin at.

My guess as to why it has a low end throttle response gain is with the throttle blade at the near closed position at idle air entering the manifold moves through the throttle body at the edges, hitting the swirl cuts on the tbs introduces more turbulent air into manifold. Open the throttle body blade more and the quantity of air running over the swirl portion of the TBS becomes less of a percentage of air entering the manifold. At higher rpms the air in the intake manifold is already turbulent and chaotic because of the increased frequency of intake strokes of the cylinders. This leads to absolutely no perceivable gains at higher rpms.

So anyway, to wrap this up, there are some noticeable gains in throttle response at low rpm which are seemingly all used up before hitting 1500 rpm. I have never heard the whistling others have mentioned and I can only guess it must come at higher rpms where the throttle blade is nearly closed and lots of negative pressure in the manifold. Most of all, this isn't something you want to spend your money on.