As far as I am aware, the sport button is a standard feature on the 2.5 si and has the effect of making the throttle a bit more responsive and the steering sharper. I have this on my 2.5si, although can't say it makes a lot of difference. The 2.5i with the sport button is not the same spec - 2.5si has 215 BHP, whereas the 2.5i (non-si) has 174-192 BHP, depending on age.
Good luck with your search.
Andy
