I remember Wheels did a test sometime ago running Optimax and equivalents in VT Commodores and AU Falcons. The Falcons showed almost no improvements in economy or performance. The computers are only programmed to run on normal unleaded, so the spark advance only goes so far.
The Commodores, and I can testify to this, did much better. We have 6 VTs at work, and when they run on Optimax they get better fuel consumption and power. Being an American design engine, they are designed for better fuel.
Maybe if you advanced the base timing a few degrees, then it might make a difference with the better fuel. Or on older cars might make the engine run better.
Ive tried Optimax, BP Ultimate, and the Castrol stuff, on my EL Ghia, and could not see any difference at all. Apart from probably keeping the fuel system a little cleaner, I dont think it would be worth it.