It's a different engine, 2.5i uses the
M54, the 2.5si uses the
N52.
Good news is that the M54 will take boost, while it's not a good idea to boost the N52 (aluminum-magnesium engine).
As roffle said either boost it or leave it alone. Chipping and other mods are just not worth it. 20" will make it even rougher on the road and you'll have even less performance.