The one in my sig (kufatech sound booster) and the one that Eskimo has can disable the startup sound. The kufatech is switched on/off via the MMI.
Although I have found that if set it to say dynamic or individual and only select it as you are driving along it will do the rev startup sound once per car ignition startup. I confess I have made on 2 occasions people jump as I passed them doing the above. Sorry Mr Parking Warden