some do some dont. mine is louder but Adams doesnt do it 
Midrange is better and the turbo spools up quicker
Tidy 
Any ideas on years, ages or anything that might be a factor to that?
To be honest the gains are good enough to go for one, just need to find a cheap one or a second hand one 
Not really sure why Adam's doesn't owl

Think his is an 03 plate, mine was a 51 and Steve's was a 52 plate.
Mind you since my turbo was replaced it didn't owl either

THS provide adaptor sleeves so ideal if you're fitting it to the standard exhaust.
Milltek you'll have to buy one separately from Jetex