[Scott]

There are much cheaper additives to increase octane rating in fuel than the off-the-shelf "boosters".

Also, race fuel is, like already mentioned, relatively pointless in a near stock street car. Unless you are running high boost and the engine has a true demand for a higher octane, then 91 or 93 should be more than adequate. Running a higher octane won't damage your engine (unless you run C16 for too long, because it is leaded), but you will burn up your wallet significantly faster.

As far as S2000 specific experience, I tried running a 96 octane/10% Xylene mixture, and didn't run any faster times than when running 93 octane by itself.