IF you can keep your foot out of it and you have it tuned right (not small carb/ big carb) AND have the properly sized turbo (both turbine and compressor size) you will get better economy cruising as the volumetric efficiency of the engine increases.
The guys at
www.theturboforums.com are of great help and there is a TON of info there that will answer just about any question you have about turbocharging.... If your a newb when it comes to building a turbo setup I would suggest a good 40-60 hours of research and studying before even considering it
Which engine are you considering putting a turbo on?