Custom modified wrx oem headers or custom modified 2.5I headers for uel. At the moment, the stock headers are not 100% equal length, and have some what of a rumble. If you extend the 4th cylinder outlet you can get a boxer rumble. Someone on lgt did it.
But in my opinion, getting headers is more of a headache vs cost. While rallisport aren't perfect, they have the least amount of issues compared to modding your own.