firstly what motor do you have? is it turbo?
if its anything smaller than 1.8 litre or without a turbo, you may find you gain a little bit of torque when you restrict the exhaust. THe 2.5" system is big for a 1.6 litre motor.

if you have a 1.8 and higher, your top end will probably lose a bit of power.
If you have a turbo, it will take longer to reach full boost.

hope that help