Originally Posted by Mann Engineering
The rear bars on the 04/05/06/07 are all different rates between model years, but all with the same fitment. This means that they made damper and spring rate changes through the different model years. The 04STI uses I believe a 21mm and the 05 used a 19mm. The 06 went back to a 20mm and the 07 went back down to a 19mm again.
I know that the 2004 STI rear bar was 20mm (not 21mm). Can't speak for the other model years though. They are definitely all the same fitment and interchangable between years, but certainly size could vary my 1mm.
The struts were changed different model years for the STI with different valving, however, it is my understanding that the spring rates and heights stayed the same for all the model year STI's which is why nobody tries to swap stock springs between model years. It is interesting that Prodrive and Tanabe both changed spring designs for the 2005 and later model year STI because the struts have less travel and don't work well with dramatic height drops.