The 850 has anti-knock sensors, so you will do no harm using lower than recommended octane. On my non-turbo, recommended is 91, minimum is 87. You need 91 to get the specified hp and mpg. However, power loss due to lower octane fuel is likely in the barely detectable range (5 or so hp). Toyota specs 91 or 87 for the same motor depending on whether it is in a Lexus or Toyota. Of course, folks pay a lot of money for undetectable differences, so I won't say "negligible".

The big question is what octane gives the best miles per dollar (euro)? It is not obvious that cheaper fuel saves money if mpg falls. Or, if you are concerned about climate change, the best mpg at any cost?

Don't trust us: Pick a fuel; run a few tank fulls; compute mpg and mp$. Go up or down a grade; repeat; compare. Cars differ depending on engine condition and conclusions based on the same data vary by perspective.

-Eric