I've been running my turbo on my car for a few days now and i can say without a doubt that with my limited time driving it my average MPG has dropped quite a bit. With all my mods before turbo I averaged 24-25MPG, now I'm averaging 20-21MPG. When I am burying my foot in the throttle and playing the car is even worse. I will be taking a trip from Phoenix to San Diego soon which is all open highway to get a good estimate of highway MPG, I'm expecting relatively similar numbers to what I was getting before at about 28-30MPG, but we'll see.
I can attest to the expense of a turbo setup, a turbo kit might cost $4-5k but that doesn't include all the other things you should upgrade with a turbo, like wheels, tires, clutch, exhaust, tune, gauges, etc. I have about $20k invested in parts in my car and most of it is performance oriented.
You also do add weight...in my case about 50lbs was added to the front end, but with 7PSI boost I'm making 255WHP, almost 100HP over stock, so the weight gain is hardly an issue. You can get rid of that added weight by changing out your stock flywheel, taking out your spare tire and all the res of the stuff in the trunk, and a few other minor weight reducing options if it's that important to you.
Last but not least, these turbo kit's do not lose responsiveness of the throttle, in fact with a lightweight pulley and clutch/flywheel installed my car revs like a sportbike...if you don't believe me:
These little FA20 engines are built for boost it seems, hell they even include bungs on the engine case for oil supply lines. It's been a long expensive journey, but if you are considering a turbo, you have to weigh all the good with all the bad. If MPG is a big factor FI might not be for you, hell any engine modification beyond a tune would probably be a bad idea if MPG is the net goal. If you are looking for more performance and can live with slightly worse MPG then FI is a viable route.