Nice thread Race Comp Engineering!!
Here's a little something I just discovered.
I was able to straight swap the factory 15mm RSB from my 95 legacy into my wife's 03 OBS. I'd removed the SB from the lagacy to up-size it to a gen 1 OB's 18mm RSB.
So it follows that the 13mm RSB from an 02-03 OBS (I assume 00-07 impreza bodies?) can ultimately be replaced a gen 1 OB's (96-99) 18mm RSB.
These are more abundant and possibly cheaper than WRX 20mm bars, so a great and easy improvement if not
THE best possible improvement.
I noticed you have specific info regarding sti part fitment etc. and those specifics are out of my experience (I'm an off-road guy with a lifted BD), but I did find something which could be of use regarding RSB on my wife's 03 OBS.
Ciao!