Down and out.
The higher the octane rating of a fuel, the greater it's resistance to detonation. As per stated earlier. The myth is that Higher octane fuel has more potential energy. That is false.
Engines like the supercoupe are tuned with boost and spark advance that would cause 87 octane fuel to pre-ignite. Sometimes on cars that can run both kinds of fuels, the owner will notice more power with the 93 octane stuff and therefore conclude that the 93 octane is more powerful. Wrong, his engine identified the higher octane fuel and tuned the engine to follow suit, thus the feeling of increased power.
I've run regular in my super coupe a couple of times. However for 99% of the daily driving I do, I don't even go past the "0" on the boost guage. So I'm pretty safe. It's when you swing the guage over the 5-10-15 psi boost side of the gauge that you are going to run into problems. If you don't feel comfortable with that, you can always remove the supercharger belt and just run "all-motor". But you will be driving around an underpowered stone of an engine.