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@ mikeBURRITO: The others here have nailed it. The fuel economy to be had from a turbo is obfuscated by a number of factors, but ultimately, that fuel economy is nothing more than a byproduct of being able to get more power out of a smaller engine.
Take my car, for instance. It is able to produce more power out of 2.0L of displacement than many naturally aspirated engines two to three times its size. The real benefit for me comes when the turbo itself is not active. Essentially, because the engine will only be at 30-40% load (turbo not active) when I am cruising, I'm getting the same mileage as a naturally aspirated 2.0L engine.
Unfortunately, when under >50-60% load, I'm getting the same mileage as those larger displacement motor. As the turbo spools, more air (and consequently, more fuel) is being rammed into each cylinder. This produces gobs of power, but also eats up much more fuel. Also, as bkrell had pointed out, manufacturers tend to run a rich fuel mixture to prevent detonation/knock. My car tends to run a 10-11:1 air fuel ratio (AFR) under boost... far from stoich (14.7:1 AFR, or 1 Lambda), and very far from lean burn efficiency (I've been told it is ~ 15.2:1 AFR or 1.2 Lambda).
On a side note, this is one of the tracks I will be going down in terms of maximizing my highway efficiency. Under freeway cruising speeds (goal is to maintain 30-40% load at those speeds), I'm going to attempt to increase my timing to 42-44* and AFR to ~15-16:1. This can be done because my turbo starts to spool at ~3,250 rpm, and my car's gearing allows my to cruise at 65 mph at ~ 3,000 rpm. Again, I'm only using the 2.0L motor, not the additional, virtual displacement that the turbo provides.
In your case, if your ultimate goal is freeway efficiency/economy, I'm not sure that converting your current motor to a turbocharged motor is the right way to go. You'd be surprised at how much one of those conversions costs to do properly, and even if done properly, your potential mileage gains are dubious at best. In my personal opinion, turbocharger kits should be reserved for those cars whose engines were designed for turbo chargers. I've seen too many blown engines and lackluster results from naturally aspirated engines with turbo kits thrown on.
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