I mounted six Airtabs just on the roof of my Smart. I could have mounted four more, for a total of ten, but was satisfied with the results just using six. Also by only mounting in the center of the roof I minimized the visibility of the Airtabs. Being a black Smart with black Airtabs they didn't jump out when looking at the car like some of the other examples on the Airtab web site.
Did they provide more stability for the Smart at highway speeds? Absolutely. No more grab the wheel with both hands when a semi passes or is just in front of the car. No more darting from side to side in a crosswind. Did they make the Smart handle like a Corvette? No, but it did make it handle like a regular compact sedan. Big improvement. Airtabs have the biggest effect on flat ended vehicles, like semi trailers, vans, Smarts and the i.
As for increasing range, don't expect any big increases, but it will help. Expect somewhere in the 1-3% range. Don't expect any increase in range at speeds below 45-50 mph. The faster you drive, the more the effect. I've owned a first gen Insight for 12 years and that car is loaded with improvements that give .5% here, 1% there, 2% over there, etc. The end result was the first car rated at 70mpg highway. I live where it routinely gets below zero and snows a lot during the winter months which drastictly reduces gas mileage. I still have averaged 62.5mpg over 12 years of use.
Airtabs are inexpensive and are attached using adhesive. This means you can remove them easily if not satisfied with the results. Hopefully someone with an i that drives at highway speeds will give them a try and report back with the results.