You don't have to really, there's plenty running the car comfortabley at 300bhp on standard turbo's, mines been running 274 for around 6 months fine now. I've heard more problems of aftermarket turbo's failing than I have of standard turbo's failing running higher maps.
There's certain maps like Turbo A and Turbo B from dreamscience which need a different turbo to function correctly, but running most DS maps, BF, Mountune, or the Code:Red maps, you should be perfectly fine on the standard turbo, and your money would be better spent elsewhere.