Since I got my IS350 a few months ago I've been doing the numbers on 95/98 RON fuel, to see how much difference it makes. Different suppliers, but usually BP, as they're convenient to my route.
I've probably put 8 tanks through it so far while I've been tracking it, and in $/km it's within 1%. I haven't had to drive in heavy traffic in the last week which means I got really good economy, so that has swung the calculation in favor of 98 by 0.8%. Before that tank it was better on 95, by 0.7%.
Bear in mind I'm just recording the average economy displayed on the dashboard at the end of each tank - I'm not recording km traveled or anything.
Interestingly, I get an 8% variance in the fuel economy just as a result of how or where I'm driving, and 10.5% variance in the fuel price. So, as long as you're using a 'premium' fuel, the way you drive and the cost of the fuel seems to have more of an impact (10 times!) than the actual fuel you're using.