To me that means the easiest power gain available is to advance the timing a few degrees and run on higher (93 for example) octane fuel.

Well not necessarily, sparkplugs fire in advance of TDC because the charge takes a finite amount of time to burn. Most of the power is produced near the top of the stroke when the piston speed down the bore is low. If your engine has an optimal burn, advancing the timing will cause the last part of the compression stroke to occur at higher pressure and result in reduced power. In other words, advancing the spark doesn't result in more power unless the burn isn't optimized and modern combustion chamber design is pretty good. Without before and after testing on a dyno, you won't know what advancing the spark does.

 

because the charge takes a finite amount of time to burn.

Well yes, but the point is the fact that the bike is tuned for 87 octane fuel, which has a faster burn, which means you cannot advance the timing for optimal power because the combustion occurs too fast and the flame front would reach the piston while still on its upward motion which would be VERY bad. By increasing the octane rating and slowing the combustion down you can optimize the timing having the spark fire earlier knowing that the piston will have enough time to just barely pass TDC. The optimal burn, therefore, is dependent on the octane rating and being that it is tuned for 87 octane there is no way the timing can be optimal for power.

 

I thought Octane numbers had nothing to do with speed of combustion and everything to do with the "diesel" effect (the higher the octane number the higher the compression that can be achieved without detonation).

 

(the higher the octane number the higher the compression that can be achieved without detonation).

That's right, because the higher octane fuel burns slower (read more controlled) you MUST use it in higher compression engines and when running high boost to prevent detonation, or pre-ignition. It's due to the controlled burn that the fuel is less likely to self ignite under compression alone. Octane is simply a measurement of a fuel's tendency to burn in a controlled manner.

 

Well, for what's concerning me and my old and pimpy t595, a couple of degrees of ignition advance gave me some better results into the low and midrange. More advance ( I tried up to 5 degrees) moves your power into an innatural place through the revving range. There will be no more balance, e.g. high revving is simply a flat, slow response to the throttle, while the torque is pulled so downwards, that the chain rattles...you could feel that engine response is more robust, but erogation will become poor and inconsistent. You'll notice pistons heavy work, building up lots of heat, that oil cannot bear for longtime. And engine itself will soon tell you it' very tired.
What you need ( and what I need ) is a map of ignition advance of your bike.....then if it's rpm related, start a light advanced copy of the original, and following the torque chart of the engine, put more or less degrees of advance where you need.....IF you need it.
Take this as a ZEN non-answer, make sure it will produce your effect.