Hi Davy - welcome to SIDC!
The 2.35 engine will handle that power no problem, but you'll never get 400bhp out of the standard turbos on the legacy. They're more or less at maximum as standard. You might get 330 but not much more. The turbine housings will take a TD04 wheel & core with a bit of porting which gives you lots of options for the compressor side. Choice depends on which particular standard turbos you have... I suspect it's a VF26 and VF27?
Before anyone chimes in and says go single turbo... YES single give potential for more top end, but the twin (or rather sequential) setup has excellent area under-the-curve, and TT Leggys (standard or especially when mapped ) are MORE than capable of giving an STI a fright! They have a character all their own... don't knock 'em till you've driven 'em