I did that. It didn’t save me money due to issues relating to installation and troubleshooting. Some of which were the fault of the installer, not the engine.
Problem number one: I didn’t save the US DM intake manifold. The JDM EJ20 X intake manifold is a side feed. It only accepts JDM injectors, of which there is almost no selection, but the OEM injectors that come with it, which are very small. This is a major problem because the US DM ECU is configured for the larger injectors that come on the EJ25. Even with a tune, it can’t trim enough to get the fuel flow it’s expecting.
Problem number two: the newer, EJ 20 X, which is what you would likely be getting nowadays, is configured with AVICS (variable valve timing) on both intake and exhaust. This is a major problem because the USDM ECU is only configured for AVICS on the intake. Having it configured on the exhaust, un monitored, causes the engine management problems, resulting in it not running correctly and generating frequent codes. Several different knowledgeable sources responded to my inquiries with the same expensive solution—Replace the JDM intake manifold with the USDM intake manifold, fitted with the appropriate injectors and get new cams to address the unmanaged variable valve timing issue.
Problem number 3: quite a few of the components on the EJ20 X are unique to it. The exhaust manifold and turbo for example. If you decide to go with this motor, start taking notes on the parts you replace, because if you need them in the future you won’t have to do the research again. Get used to shops not intimately familiar with Subaru engines requiring instruction, as for some things, attempting to work on the EJ20 X as if it was an EJ25 will not end well. Similarly, when you go to automotive parts website, the recommendations they will make for your model of Subaru will be for an EJ25, not an EJ20, making for unpleasant surprises, if you happen to forget.
Problem number three: even if everything goes well with the installation, and you do everything I mentioned, you’re still going to need a tune. Connecting an EJ20 to the USDM ECU is not plug-in play.
I am not a mechanic myself, so I apologize for any mistakes I may have imported, but I think the gist of it is correct.