I think this is something
I think this is something that is just going to have to be tested. Personally, I don't think the turbocharger is going to help at all with mileage. Not only will it require nearly unattainable discipline to NOT blip the throttle and become drunk with the awesome feel of your car tugging [can you tell I used to own a turbocharged car? =P], but I'm a true beleiever in more air = more fuel = less mileage.
To get the best mileage, I do think we need to look at efficiency, but we need to look at it with using the least fuel. You can create an efficient setup that drinks gasoline.
I think if you're dead set on using a turbo setup, look at getting a small one [possibly much too small for your engine] and using it beyond it's efficiency zone. You do not want to create compressor surge, but if you choose a compressor that plots your engine far from the surge line and way out of the efficiency islands of the compressor map, then you'll be blowing 'hot air' into your engine. This is the only line of reasoning I see as beneficial to having a turbocharger on a car attempting to attain good mileage. I am no guru on turbos, but I have owned one, and do know a little.
As someone else said, plumbing and the rest of the stuff is very difficult as well - the whole ordeal is really a headache.
One thing to caution - if doing as stated and blowing hot air into your engine, you begin teetering on the razor of awesome efficiency and catastrophic destruction. There is a very, very fine line there.
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