Agreed, the stock EJ setup has a tonne of pipework to pressurise. Hardly the best design for spool. That's why the R35 GTR has its turbo hotsides cast into the exhaust manifold, likewise the turbos on the current VW Golf GTIs and Rs. Low mount turbo on a Subaru is $$$$, needs heaps of custom pipework, lines, scavenge pump, etc.
I was planning on getting a JDM GRB STi when I get back to NZ next year and do some upgrades to it. Want to keep the spool and transient response as close to stock as possible, but be able to use the entire 8000RPM of the EJ207 without running out of puff. I was planning on seeing what Steve Murch could do to upgrade the stock one, but given what you've said (no ability to upgrade to BB CHRA, limited options for wheels and housings), I might be better off to go straight one of the available aftermarket twinscroll options. Pity none of them are cheap. Thinking Borg Warner EFR 6258 (requires work to fit) or possibly the Litchfield LM400 (but that's $3k+). IHI RX6R would be wonderful but the price is insane. Other suggestions welcome.