So I'm really curious about the pistons. That could either be no big deal or big freaking deal when it comes to turning the power up. Why swap them? The biggest reason I can think of is cost-savings OR greater rotating mass efficiency through a different crown design or ring pack design. But it's not like the regular n20 is some inefficient beast - so neither of those really makes sense to me except perhaps cost-savings achievable by using a lower quality piston because you are running so much less dynamic compression against them with the detuned engine.
If I'm totally off base then the 320i is going to be the entry-euro-tuner's delight. No forced "lines", cheaper, and an engine ready to play in the ~260whp range on pump gas.