I'm no expert, but I wouldn't run low octane with 13:1.
Bikes are high compression, and run hot. That's a recipe for ping and detonation with low octane gas. The 100 will not help if it isn't needed, but it will take away some power.
Whoever said it above was partially right, the octane rating is mainly a way to reduce or avoid detonation, but can affect power as well.
The lower the octane, the more explosive the fuel will be. So, for a given engine (compression, stroke etc.) the lower the octane the higher the power; however, if compression is high enough, the heat and compression can cause pre-ignition, and instead of a controlled burn wave front, you will get small explosions (ping). If the entire mixture explodes, you will get detonation, which can quickly kill any kind of piston.
You want to run high enough octane that you avoid ping, but any higher won't get you anywhere and you may end up losing power.
I laugh when people put 93 or similar in their naturally aspirated, low compression truck, thinking it will 'clean out the engine'. Its the opposite, 87 will burn hotter, and will do a better job cleaning (now that all gas grades have detergent additives).
Hope this helps, sorry its so long.