I love it, but yes when you gun it it absolutely sounds MUCH louder for obvious reasons. look at the giant hunk of plastic you took off, it is a giant baffle system which was designed to cut down on noise, which then cuts down on airflow. You replaced that with a straight pipe with no baffle system so you get more air, more power, and better MPG. But it WILL be louder, only when you gun it though, not in normal city driving conditions, just at or above about 4,000rpms.
I love mine, but I do understand how some people would dislike the sound. It can be loud...