It has to do with the compression ratio of an NA motor being higher then that of a turbo motor, resulting in a higher likely hood of detention on pump fuel even at the same boost levels. But in saying that there is power to be made from de+t ing an RB25.
Hell my mate had an old dirty thirty Natro, bolted on a ling long spec 35 and made great power for what it was, has just purchased a water meth kit for it and with more boost wound into it should see fairly decent figures...
I'm not going to say this is the best option but it definetly is one (just have a read through that thread you'll see)