"Can I install a turbo on my <insert non-turbo Subaru model here>?" This seems to be a common question I'm fielding lately, so I'm going to gather my thoughts here, then just be like "Read this link."
So you've got a non-turbo 2006 Subaru Legacy, or 2018 Impreza 2.0i, or 2012 Crosstrek. You see these cool turbo Subarus rolling around and you think, "Why can't I get some of that? Subarus have interchangeable parts, right? It's like a Lego kit, right?" Well, as with most everything else in the world, the answer is that it's a little bit complicated.
Basically, in almost every case, it's going to cost WAY more than your car is worth to turbocharge it. You would be better off just selling your non-turbo car, and purchasing the equivalent factory-turbocharged model. I know, you have grown attached to your existing car, and you added those custom Hella Supertones horns, and you don't want to do that all over again, but trust me on this.
Let's pick an example just so I can prove my point. The 2006 Legacy owner wants to turbocharge his car. On the one hand, we know this is possible to do because Subaru offered a turbo version of the car in 2006, the Legacy GT. So the parts exist, and it could be done.
Now let's go through what you would need to do it. At the bare minimum, you would need these components:
Probably the turbo model car has additional oil cooling that the non-turbo model does not have, but we'll ignore that for the moment. This is already thousands of dollars in parts, and we haven't even tallied up any gaskets, miscellaneous clips and trim pieces, plugs, clamps, nuts and bolts, and other random bits. "I'll get all these at the pick-and-parts place for cheap," you say. Sure you will. When's the last time you saw a Legacy GT in the salvage yard? Yeah, I thought so.
Oh, and your non-turbo clutch is probably not going to be happy with the increased torque, so plan on installing a higher-strength turbo car clutch.
But anyway, let's say you DO assemble all this hardware, and you bolt it all to your existing non-turbo block. That 2.5i block has a compression ratio of 10.0:1, which is much higher than the equivalent turbo block, which was 8.2:1. The higher compression ratio will make the motor more prone to detonation under boost, meaning you will have to run less-aggressive boost/fuel/timing settings than you otherwise would be able to, so the maximum power will be lower than the turbo motor would generate.
Oh, and speaking of ECU calibration, how are you going to accomplish that? The easy way would be to use an Accessport and a turbo-car ECU, and have it custom-tuned on the dyno, but then you'd have to acquire a turbo ECU (more money), and is the ECU wiring harness identical between the two cars? They are almost certainly not, so you would need to re-wire the ECU wiring harness. That sounds like a lot of difficult work, fraught with the possibility of difficult-to-diagnose errors. Okay, so let's stick with the non-turbo ECU. Can that even be programmed to handle a turbocharged motor? How would we even program it? There is no Cobb Accessport for the non-turbo ECU. You probably could have an expert program it using open source tools, but the number of people who can do that (or want to) is small, and they charge a lot.
"My local JDM importer place has all these EJ20X motors for pretty cheap. How about I use that?" The EJ20X is a dual-AVCS JDM (Japan Domestic Market) motor that was never offered here in the U.S. (AVCS is Subaru's variable cam timing system. Dual AVCS means both the intake and exhaust cams are adjustable.) The only ECU in the U.S. lineup that can run dual AVCS hardware is from the 2008+ WRX STI, and that's not going to be plug-compatible with your 2006 Legacy 2.5i body harness, nor would the US ECU work directly with the JDM cam sensors. Again, it COULD be done with custom wiring and/or sensor mods, but it's not simple or cheap. We have seen these engines installed with the AVCS locked out, but the result is LAG, since the cam is extra peaky. It was designed have a tall torque peak that can be moved around as the engine goes through its RPM range. If you can't move it around, you get a very weak low-end and an unusually fast fall-off after the torque peak.
Now let's go back to the value of the car, and what you might purchase instead on the used market. Looking at a standard used-car value site, I show a ballpark value of a 2006 Legacy 2.5i manual-transmission sedan at something like $3800. If I change it to a 2006 Legacy GT manual-transmission sedan the value goes up by something like...$300. I know what you are going to say, "I can't find a Legacy GT to buy." They are out there, but they were never common, so be patient and you should probably cast a pretty wide geographic net. Also remember that the Outback XT was also available for that timeframe. You could even get a manual trans Outback XT.
So my point is for a whopping $300 difference in value, you could just upgrade your existing Legacy 2.5i to a factory-turbocharged car that is ALREADY BUILT. Why would you bother spending $5000 (or more?) converting your non-turbo car? The whole car is not even worth $5000.
Now I've offended that reader out there with the 2018 Impreza 2.0i. That person is saying, "Hey, my Impreza is worth way more than that! It's gotta be worth it to turbocharge this beauty!" My point still stands. Just eyeballing used car values, let's say you can get $19,000 for that Impreza on the used market. You can get a decent 2018 WRX for maybe $23,000. There is NO WAY you could turbocharge the Impreza for $4,000. Even if you did or could, the WRX has SO many other factory upgrades -- wider bodywork, bigger brakes, bigger wheels and tires, performance suspension, the list goes on.
Of course these swaps have been done, and you can find examples of WRX-swapped Crosstreks, STI-powered base Imprezas, and other fun projects on the internet. Trust me, those projects MADE NO FINANCIAL SENSE. Sure, if you love a challenge, have tons of free time, and you hate money, jump right in. Otherwise, if you are a sane and rational person, just sell your existing car. Buy the closest Subaru factory-built performance variant, and start your project from there.