That answer is dependentent on a few different factors, mainly being how much mods you have done or will be doing to increase horsepower output to the rear wheel.
The more tourque being placed on the wheel translates into greater stress factors on your chain. With a smaller diameter front sprocket, you are increasing the movement of the individual links as they rotate around the front sprocket. These two factors combined, equates to significantly higher wear on the chain.
A lot of guys running turbos were actually increasing the size of the front sprocket.
My suggestion would be to increase the size of the rear sprocket first.