Most who know would tell you the EJ22T is the best motor Subaru has ever built. It's closed deck, has oil squirters, IIRC the crank is forged and the rest is high pressure cast (one step below forged). So stock, the EJ22T internals are good for about 350 chp. So you can really crank it up with a little work.
If you are going to do phase II heads, head work, get stand alone engine management, larger injectors, fuel pump and a large FMIC then your t3/t4 will probably work out pretty good for you (although it may be a little laggy). As long as you go into this knowing you will need to custom fabricate most of your intake plumbing and exhaust. Or if you may want to try and resell your Garrett and look into a nice slightly used IHI turbo. Then you could use some of the extra money you'd save on an up-pipe and down-pipe towards the swap, head work ect.. but you probably won't be looking at as high of numbers with the IHI turbos.
What are you're goals with this build? What kind of numbers are you looking to get?