I looked but couldn't find any threads for HP mods for a blown 4.2. There must be some threads about loonies seeking 400+ hp.

 

Sadly, maybe because there isn't very much available. The commonest I've seen are: smaller supercharger pulley, bigger crank pulley and porting of the supercharger.

Some companies do a re-map of the ECU, and some also have thoughts on replacing the supercharger altogether with a different (more efficient) type of supercharger, but haven't seen reports on the latter yet.

 

Reflashing on most turbo cars can make crazy hp if you can get the flow, cooler and other parameters in concert. My Forester is easily pushing 265-275 with just up/down pipes form turbo and a reflash, that's up from oem 215bhp.

My guess is the 4.2 is way understressed in orig form and could handle more input reliably. Not sure about the rest of the driveline tho...