How does a 535i respond to different fuel octane ratings?

[pundit]

Is there much of an advantage in running a 535i on fuel with any higher octane rating than say 91? In Australia we have fuel with the following octane ratings...

Regular Unleaded - 91 Octane
Premium Unleaded - 95 Octane
High Octane Premium Unleaded - 98 Octane

While the Vanos version of the 525i would undoubtably benefit because of it's engine management system (knock sensing etc) would there be any gains with a 535i? For example 95 octane costs around 10% more than 91 octane so switching to 95 octane one would need at least a 10% improvement in economy/performance to offset this. I assume this would require the 535i to be retuned ie. have it's ignition timing advanced to make use of the higher octane. I have tried 95 octane in my '89 Toyota van (fuel injected - no knock sensing) and the result was less power and a reduction of about 10% in fuel economy compared with regular 91 octane! While I didn't alter the ignition timing, (it had already been pinging very slightly under load whilst on 91 octane), the pinging stopped on 95 octane but everything else went backwards! Does the EAT chip require the use of higher octane fuel?- Cheers.