Oh... WOT surge, not shut off surge. Depending on what's causing it, more boost will definitely make that worse. In fact, it'll probably just surge at the same point except possibly more. That's bloody weird you are getting it with -5s on a RB26. I've never heard of it (but I don't know much). It needs the engine to ingest more (revs come up), ie. the exhaust wheels to flow more, possibly actuator issues/wastegates to even the in/out of the system. Or, like you suggest, maybe the intake is restricting it somewhere throwing off the balance.
Can you describe at what RPM it comes on... I assume it happens reasonably early on then the engine 'catches up' and ingests the air which balances it?