Your octane boosters like hot 104 are just petroleum waste products mixed with kerosene packaged in a pretty bottle.. refiners have to do something with the leftovers from refining gasoline, and hazmat is too expensive, so they sell the waste to companies called converters ( STP is the biggest).. if the stuff was any good, why not leave it in the gasoline to begin with?
As has been said, the octane increase is minimal, but the other downsides are btu loss ( from the lower energy kerosene ), metallic deposits and emissions you get when the mmt is combusted.. oh yeah, you'll also get a sticky sooty residue that does wonders for ring sealing and your cylinder walls.
Tom