Who knows what's possible, but I think it is safe to say that you probably don't.
If he gets bored with the power I will suggest that he goes to one of the sydney tuners that specialise in NA performance as they will have a better idea of where the limits are.
And if that isn't enough I have routed all the wiring for the Vipec specifically so that is away from any turbo(s) fitted.
You need to go and do some sh!t to your car, you spend too much time on here being bitter.
I have been doing stuff to my car. Though not many are interested in Holdents on here...

The only reason I'm a skeptic is because, when I look at what other NA 3ltr engines are capable of, I find very hard to beleive that the Subaru variant will be much different.