As everyone else has said, it's only when the engine is working hard that you get a great but louder sound and it's NEVER boy-racer.
I've had my Non-Res Hi-Flow+Downpipe Milltek on now for over 40,000 miles. I've now got Stage1 Revo and quite a lot of other performance mods. At first it was boomy around 3,200 revs for a few weeks and then matured past it. Combined with an aftermarket Air Intake and a remap, it'll deliver more power but will add nothing much without the remap. Throttle response gets improved with or without a remap.