Are Base Model Impreza/WRX Aftermarket Parts Interchangeable?

[vitallynick]

Quick disclaimer first, I don't know my way around flat 4 motors very well, so my knowledge on Subarus is very limited. I bought a 2011 Base Model Impreza recently, and I've been having trouble finding certain parts to modify it. The parts I find always have applicability to the WRX but more often than not don't mention other applicability, I was recently looking for a new exhaust manifold/downpipe and could only find ones for the WRX and STi. are some aftermarket parts on the WRX and Impreza interchangeable? (i.e. do they have more or less the same engine, with one being N/A)?