The NA EJ22 makes enough power to break the R160 rear diff. I did in a few with mine. Little bit of wheel hop and BANG it's over.
You will need to take the rear transfer housing off the transmission, which can be done with it in the car. You can either weld up the spider gears in the center diff, or you can buy a spool to replace it. That still leaves all the power going through the rear transfer gears in the back of the transmission which aren't that strong. Again, I shredded a couple sets of those too.
If you plan on driving like a pansie, then the car will hold up fine RWD. If you beat on it like you're going to when you're having fun, it will break easily.
If you want to do RWD right, there is a conversion bellhousing sold to bolt a W or R series Toyota transmission to a subaru motor. That fixes the weak transmission side. Then put a R180 out of a STi or a R200 out of a nissan in at the rear and have custom axles made.