On the 1.5, Performance mode only increases the temperature threshold for the battery before it steals power to run the a/c. On the 2.0 and 2.5, Performance mode does actually increase the allowed amperage draw. On the 2.0 and 2.5, you do need to engage Performance mode to realize the rated 0-60 time.
So, no real performance difference on some cars, but there is for others.