I definitely think DT (or even the end user with some knowledge of the ECU tables) could do a MPG map. The MS3 guys create multiple maps for mpg, drag, DD, track, etc using the Cobb AP - and the mazdaEdit software is generally more powerfull than the AP (from what I can tell without actually using it), so I see no reason why not. Though I have to qualify this with the understanding that I'm not an ECU tuner (though I'm learning more every day) and haven't tried it myself.
I would love to be able to provide dyno plots before/after (I also agree about the area under the curve part), but I can't justify that expense to myself when it's mainly for curiosity's sake, even though I love numbers ad data of all types, lol.
IMO people usually over-exaggerate the difference between regular and premium without doing the math (not accusing you, just sayin'... most people). Note: all of the numbers below are backed up with data that I can cite sources for, if you don't believe me. (rules)
I currently drive about 13,000 miles per year, and I expect that to go up to 14k this year
My current average mpg for the mz5 is 24.98 mpg. This is tracked using actual miles traveled and gallons pumped - vehicle computers are inaccurate and almost always overestimate the mpg. i.e. don't trust what your car is self-reporting to you.
The current difference in price between regular and premium in my area is $0.319. The difference has been under $0.30 the last two years, and recently spiked slightly to ~$0.32, but we'll use $0.32 (source:
US Energy Information Administration You can graph the info on the webpage, or download it in .xls form and manipulate it yourself.)
14,000 miles at 24.98mpg = 560.4 gallons
560.4 gallons at $0.32 extra for premium = $179.34.
So it costs me an extra $179 per year to use premium (about $15 per month, or $3.45 per week). I'll gladly pay $179 in fuel for 10hp. Some people might not.
As you mention, it's not unusual for an NA tune to actually improve fuel mileage and gain power at the same time. Obviously better mileage would reduce this $179 figure.
We'll see if I have the NA tune on the car long enough to collect enough data (large enough sample size) to judge the tuned NA mpg, before I go FI.