My cars have *always* gotten better mpg (usually at least 2 mpg or more depending on car/engine/weather/etc.) with Amoco & Sunoco (neither are top-tier), Shell, and Chevron than with the other brands, e.g. Pilot, Flying J, Mom & Pop, Citgo, and around here, places like Kocolene, Moto, Thornton's, and Speedway/SuperAmerica.
If I would normally get 25 mpg, and I can get 27 mpg with better gas, that is an 8% gain, so in this case I could pay 8% more for gas and break even.
1. At $3.00/gallon, I could then pay $3.24 for better gas and probably have a cleaner fuel system to boot.
2. Rarely is the "better" gas 8% more...it's usually more like $.02 to $.10, so that means you would only have to gain .67% to 3% mpg.
3. For my money, it's better than a break-even: I'm money ahead, so it's WELL worth it! Of course, your results may vary!
(This is *my* logic, skewed as it may be.)
I don't think "engines holding up", as Master Acid mentioned, is really the issue of concern. It's more a matter of clean fuel systems and economy. I know mine is better because I've checked, literally, every tank of gas I've bought in the last 20 years...literally...did I mention literally?
Here's an interesting article that some of you may have seen...to me, it was very well-written by a highly-credentialed author and quite compelling . He doesn't mention "top-tier" specifically, but he does mention the quality of gasolines.
http://vettenet.org/octane.html