depends on what mods you have on already,
i think the honest thing to say, is you won't gain masses of BHP from it, but, and this is a big one, it greatly improves how the car performs, so you want have as much bragging rights in the pub, but she'll ahndle and accelerate a whole load better, and make use of what you actually have.
if you want big gains you have 2 choices, go FI or buy a faster car