club.liberty.asn.au

http://www.youtube.com/watch?v=T1sSSd8ig9Y

The Legacy/Liberty you see in the video is most likely a Non turbo, Automatic, 2.5i.
Would anyone have any idea what year model impreza UEL headers they would have used to get that boxer rumble?

The turbo is in a completely different spot on the Gen 5's.
You would have to change all of the Turbo plumbing to fit Gen 4 UEL headers (major $$$).
As the Gen 5 turbo is behind the engine in the middle, you would have to get custom headers made.
The Gen 5 exhaust manifold has been designed to be short and equal length to reduce lag, make the engine smoother and more powerful.
By putting unequal length headers on, you are increasing lag and reducing power
When the Subaru boys get serious about power, they go for equal length headers.