I would say the best head you have with the stock cam if that combination is possible would probably get you the best mileage, turbo or not. Improved head efficency as long as it isn't only for high rpm use by enlarging the ports excessively can help mileage out I would say. Any time you make the air flow through the head smoother that has to help the engine get better mileage. Just don't enlarge the ports so that it has bad flow velocity at low rpms where you are trying to get better mileage.
Also higher compression can help but don't make it run on race gas only or you won't really be saving money
I haven't really tried any major changes to an engine to get better mileage so I could be off on what I say.