Well first of all are you going to use the EJ22 block from a NA legacy or the EJ22T from the older turbo legacy. If it's the EJ22T there about $1,500 brand new. I don't think you want to use the NA block.
The EJ22T has a closed deck, oil squirters, 8.0:1 compression and you only lose a small amount of displacement. These hybrides have been succesfull with Paul Eckland's rally cars and others, but I'm not sure if Paul does any internal mods to the engine. The SCAA rules would apply there. When you mate the EJ25 heads to the EJ22T block the flows aren't optimal, but enough for the application you're looking for.
I've done the research for both routes. If you wanted to use your EJ25 motor you'll change the pistons, rods and probably the springs, valves, and cams to get the most out of the engine. This setup will smoke a hybrid, but it's $$. Also, it'll cap around 400HP due to the open deck which can be welded closed for about $1000, but it's difficult to find someone who can do it.
All prior information is from research not actual experience, so I could be wrong.