There is a HUGE misconception about what the difference in Octane is.
Octane rating is based on a 100% scale. The percentage of iso-octane/n-heptane is the octane rating. for Example 91% iso-octane/9% n-heptane is 91 Octane.
This however, is not how it is ACTUALLY measured at the pumps. Other fuels and additives (Like ethanol) can change the octane rating, so 91 octane means that is has the knock-resistance of pure 91% iso-octane fuel. There are also different ways that octane is measured. RON, MON, AKI, or (R+M)/2. (R+M)/2 is the most common in the US. Not all measurements are equal to each other. For example a 95 RON rating is only a 91 (R+M)/2 rating.
Ok...so now that you got that useless information...what does a different octane rating mean to an engine?
Higher Octane = Harder to ignite (by the spark plug, or detonation).
The spark plug igniting the fuel is what you want, detonation or pre-ignition is not what you want. Pre-ignition is when the air/fuel mixture is set off before it should in the engines timing cycle. This can cause severe engine damage/failure. This is usually caused by extremely high air temps, timing that is off, or LOW OCTANE fuel.
The engineers who have designed the 250 engine put HOURS and HOURS into research to figure out what kind of pre-ignition resistance (octane) fuel it needs. The only reason to use higher octane is if you have modified the engine for higher compression, are running forced induction (effectively making the compression higher), or drive the bike under extreme hot conditions (where air temps would be really high). I run 91 octane because it is hotter than satan's ass crack here in the summer and I would rather not chance damaging the engine.
Higher octane fuel DOES NOT mean more power, infact the opposite. When fuel is harder to burn....less of it gets burned, meaning less power. The amount of difference between 87 and 91 (R+M)/2 isn't much so my small loss in HP is worth the safety I gain from it.
Other than octane, there are companies that apply additives to their "ultimate" or "premium" fuel, that they don't add to the lower octane fuels. The main thing is detergents to help keep the fuel system clean.
I hope this helps clear up the debate a bit.