You don't need forged pistons for 300hp internally. You just need to make sure your engine is tuned properly and doesn't run lean.
Stock pistons are not forged but they are Hypereutectic type. These are much stronger than standard pistons and slightly less stronger than forged. Barring detonation or running too lean, they should be good for 300hp easy. But if you feel a need for a comfort zone, forged is the way to go. Though I believe forged t-bird pistons are a custom order.
The weakest point in the SC block is the rods.