Ok here's my take on it.
For my 94 Ranger with 240,XXX miles on it: it doesn't make one sh*t of a difference, so I go with low grade. I only use it for wheelin' anyway.
For my 99 Corolla with 63,XXX miles on it: I can only get it up to ~103mph when I am on low grade. On high grade, I make well above 110mph. I don't know how much above, because the clock only goes to 110. I use high grade for that alone, plus I don't want the engine to knock (not that I know half as much as some of you do about the matter). When I drop in the turbo I have sitting in the garage, I will stick to high grade.
For my 06 4Runner with 11,XXX miles on it: I use high grade because it doesn't cost a whole lot more (maybe 25 cents a gallon max), and I feel like the whole truck performs better when I do. Plus, look what it does for my Corolla lol, why the heck wouldn't I. Now on the 4Runner, I can get past 110mph without breaking a sweat...even on low grade. But I figure, I'm probably going a lot faster on high grade if what my Corolla does is "normal". Again, I'll stick to my high grade when I put the turbo system in. And the manual says "for better performance", so I figure that's a good excuse too.
Oh...and my mommy pays for my platinum credit card. That makes a big difference too