plan on taking on the challege of designing a rear air diffuser (similiar to the 06's) for the previous years of the wrx/sti?

Varis had the diffuser even before the 06 came out, but it's mucho dinero. I only know it fits earlier versions so i think Varis advantage is that they bolt on properly to the earlier models without drilling your trunk space for the spare cuz that's one of the things you have to do to our cars to fit the OE ones, i believe part #91225 FE000 is the one on the '06 and it's less than $70.00 according to posts in another forum.

One more thing, Varis is much bigger in area coverage, the OE only covers prolly 1/3 the area underneath.

Cusco just came out with one that looks very similar to the stock 06 diffuser.
http://www.cusco.co.jp/05product/diffuser.html

I'm not sure if anyone is selling these yet though.

i believe that is the oem 06 diffuser. what they are showcasing is the chassis bar... IMO

Oh yeah, I think you are right. Sorry about that!