Sorry for not getting back to you sooner, I'm not so active on this forum these days. If the logic is, "this DI system wasn't optimized for a turbo, it's easier to just get rid of it when you go that route if you can control knock" then I agree with that. If you're arguing for deleting the DI on any but the most extremely modified n/a applications I just don't agree with that. This combustion system was designed for n/a operation.
A few points:
1) if you've got a boosted engine with something suppressing knock (straight E85 for example), the DI is probably superfluous at best and a liability at worst.
There's a lot of mixing and wall wetting problems avoided by just getting rid of it.
2) Whatever this other person who turned off the DI in the ISF did, we don't know what injection timing he was running on the PFI injectors. If that was more optimized his problem could have potentially been avoided.
3) Toyota already has a turbo DI version of D-4S in production. It's in the Lexus NX. It's significantly different from this engine.
4) Knock limit is mostly opinion, unless you can hear the engine rattling with the naked ear. Then it's self-evident.
You would need to provide details on how you set your center frequency, gains, and other filtering parameters. "It knocked less because Motec said so." With the stock ECU you are trusting the decisions of whoever set the signal processing at Subaru, but now with an aftermarket system it's just guesswork at best.