A custom turbo for the 2.5 may get you close to 250bhp, but gains will be limited to the CVT's power limitations, and the engine's internal limitations. Achieving 500bhp on the 3.5 will not happen with the 6spd or CVT either.
The Altima simply isn't the platform to use if you're a serious modifier and/or want serious power gains. The 270bhp on the V6 is actually plenty fast and will smoke a lot of cars from the line or on the highway due to CVTs ability to produce instant torque/power. It's already difficult to force induct the 2.5 to achieve 270bhp.
