To elaborate on why it can be bad to run such high octane gas on a low compression motor, let me make sure you know why you need higher octane gas on a high compression motor. Let's say you have a 12:1 compression ratio. Now on the compression stroke, the air/fuel mixture is going to be compressed to a volume 1/12 of what it previously was. So let's say at 180 degrees BTDC, your cylinder volume is .4 L. That means at TDC, your cylinder volume is .033 L. That's a big difference! Now during compression, a lot of heat is created due to the immense friction created between the air and fuel molecules which are now bouncing off of eachother at a much faster rate with such a little space in which to do so. High octane fuel takes more energy to combust, which assures the fact that it will not combust due to the compressing of the a/f.
Now to the problem with using high octane fuel in low compression motors. (I dunno the CR of the Z6, but this is a general explanation).
Imagine again your .4 L cylinder volume. Now with a 9:1 compression ratio, the cylinder volume at TDC is .044 L. It doesn't seem like much of a difference, but in reality, it is a 25% difference! Now with that 25% difference, there is a lot less energy being produced due to friction (I don't really know the proportion--someone feel free to insert it here). Now in the high compression motor, combustion could be occurring due to BOTH the spark from the sparkplug and the friction. But in a lower compression motor, the energy created by friction is not present (or at least isn't as strong). So some of the fuel may not combust properly. It then falls to the piston, and is not combusted on the next stroke (as more unburnt fuel is added). As fuel pools unevenly, it can unbalance the engine, and make engine emissions more rich due to occasional combustion of the pooled fuel.