My verdict: a nice placebo part wich does not give any power increase but gives you a better sound from your engine which gives you the impression you did gain some power...
my thoughts exactly.
can i ask what year your JTS is? it seems after march/april 2003 alfa corrected the problems on the JTS engine so if it were say 53plate the factory out put should be the full 165bhp and i'd imagine tuning this would give much better gains than you achieved providing yours is a pre 2003 model.