You'll get a load of people now tell you what dB they tested at. Truth it, it's all nonsense as most results will vary day to day and TDO to TDO. It shouldn't, but it does.
Get yourself across to Snet when MSV are running a day, they'll test you static for free and then you'll have a rough baseline. Most people now fall foul of driveby noise though, which is impossible to guess: I've seen a Viper be borderline static but clear driveby, and a quiet R32 smash the driveby on the same day at Bedford!
That said, if the ART pipes are test pipes then it's likely you'll fail. Most decats are too loud. Also, given the NIMBY attitude towards motor racing these days, we have a responsibility to make our cars as quiet as possible to not give them any more ammo IMHO.