I noticed that the 04' US Impreza WRX STi had a nice little air foil on the drivers side (LHD) windscreen wiper. I took a close look and saw that it was the same size as the one on my Forester. I did a similar retrofit on our 97 Outback Limited when I noticed the 98's OB had a foil. The foil helps keep the blade in contact with the windscreen in cross winds and at speed. In this case it was the wiper blade holder that had the foil. The swap was quick and easy.

I purchased the blade from here:

Lisa W. in Parts at Subaru of Gwinnett 800-584-8727

Part number: 86542FE020 - Blade 550 W/FIN LHD

To remove the OEM blade tilt the arm away from the windscreen till it locks in place. Rotate the wiper blade CCW (or anti-clock wise for you chaps overseas) till it's at a 45° angle to the wiper arm. Press the little tab to unlock it and slide the blade down toward the windscreen. Install the new one by sliding it up in the hook till it clicks. That's it, pretty simple.

Close up of part number:

Removing Blade and holder from arm:

Close up of the tab to push:

Installed (sorry car needs a good wash)

close up of blade installed

I just replaced the rubber wiper part in the STi wiper blade assy. You can get just the OEM Rubber:

PN: 86548AE150

This PN is for just the rubber replacement. I got mine from Subie Gal at:

www.allsubaru.com

The rubber just slides out, it's a little tricky to keep the flat rod stays in place but after a little fiddling it works fine. Below is a scan of the change procedure. It should be in the owners Manual too: