The front piece looks like it'll increase frontal area, not all that good a thing.
Remember that it's the driver, not the car that accomplishes mileage. Working on technique is going to get you more progress than anything else, and without technique anything you do to the car is wasted.
Your first hardware mod should be a ScanGauge or an UltraGauge- because without information, you're only guessing at technique. Besides, if you can afford a body kit, you can afford to buy both gauges, learn them both and put the other one on CL.
1. Get instrumentation.
2. Read
109 hypermiling / ecodriving tips
3. Profit!!!
Once you've settled in to an average, predictable mileage, modifying the aero is less of a complete waste of time and money. Try it then.