Hi, I;m also looking into the exhaust system on my car, I have come across this... The middle pipe is about 3" longer on the forester than that of the impreza, but the s/sl impreza cat back system I ordered will fit. it just needs the rear box hangers moving and a 3" insert made up to make the length correct.
I did read somewhere that some early impreza mid pipes are the same length as we require.
Let us know how you get on and I'll do the same.
D