For the 02-05 WRX's, it was fairly common to replace the downpipe and run factory maps. The EJ205 has smaller area piston heads (same throw), and a different computer/fuel delivery setup. There was more slack in the system and factory tunes were very rich. Cars typically ran better, cleaner, and generated more power with a downpipe+stock map than they did from the factory. The newer cars are DBW instead of DBC, use a different MAF model (open loop vs closed loop issues), and have a higher displacement from their larger piston area (more sensitive to lean conditions). Because of this, they should be tuned for.
2015 STi: Still Stock
04 STi, 04 WRX: SOLD