I have heard dump valves are good on turbo engines, can I fit one to my Smart?

No, it is not recommended, as there is a formula for determining the size of the aperture in the dump valve. Thus, most dump valves simply dump too much boost for the Smart instead of preventing the turbo ‘stalling’ (stopping as boost backs up in the system as the throttle is shut), it dumps too much boost causing the turbo to over speed risking damage to the turbo bearings.