1994 Mustang GT gas question?
Don't fall for the the common delusion that higher octane means higher performance. That's backwards. It's higher performance engines that need higher octane.
Higher octane just means the fuel can tolerate a higher compression ratio without detonation ("knock") a condition where the fuel/air mixture starts to burn before the spark fires, from the heat caused by compression (like a diesel).
Unless you raise the compression ratio of your engine, it needs no higher than 91 octane to get full performance. If the owners' manual says 87 octane, that's plenty, but most cars now need at least 89. Any higher octane would be treating a problem you don't have, like giving antibiotics to someone who has no bacterial infection.
A high performance chip may alter the ignition timing and fuel mixture. If it advances the spark, then a higher octane gas may be in order. If it richens the fuel mixture, then that won't change the octane requirement. The chip manufacturer will tell you what octane gas to use.