not much, and it isnt insanely loud. that depends on the muffler you choose to use or not use. I ran 2.5 unresonated with just a "turbo" muffler from summit. it was fine. Open exhaust, it'll be pretty damn loud, but even then... its not that loud till your really on it.
I ran 2.5 on my na 1.6 for a long time. just do whatever you want. you'll be happier in the end. dont ask here or anywhere else, just go and do it.