No need for anything higher than 87 unless you have raised the compression ratio from stock. High octane levels are only needed for high compression engines.
To spare the long explanation of what octane is and what it does know this, gasloine has an allowable range of lengths of carbon chains that can it can comprise. However, octane is more stable at higher pressure than other length carbon chains. So if you run relatively low octane gasoline like 87 in a high compression engine it tends to ignite on its own due to the pressure increase before the piston reaches TDC (before the spark plug would normally ignite it) causing an audible "knock" sound and can lead to major damage such as bent valves etc.
Long story short, a stock Busa is considered standard compression and can be run on standard 87 octane gas.
BTW, the higher the octane rating number, the higher the ratio of octane is versus the remainder of the carbon chains in a particular gasoline. And yes octane is 8 molecules long (hence the "oct" latin prefix). Similar simple compounds that are actually different length carbon chains include propane, butane, methane, etc. Bet you didn't know that the difference between octane in gasoline and propane in your grill is the length of the carbon chain. Cool huh? And these are all "distilled" or refined out of crude oil along with motor oil etc. etc.