Well when I put in the K&N drop in I noticed a little diff. Then the TRD catback and I noticed a big diff. Next was the aFe CAI... It sounded throughtier and a little pepier at higher RPMs. At lower R's it didn't have what I would expect. It even seemed to not down shift correctly and bog. After I installed the URD cali it had the get-up-an-go I expected. It comes pre-programmed with a tune specific for the aFe intake.
I have a modified RX7 and know that all the bolt on crap in the world won't make your car really fast with out a tune (Programable fuel computer). I would have liked to get the APR X1 like Skyline, but I just can't justify spending that amount on the T4R.
The URD was well worth the $200 and really easy to install.