I don't think running higher octane does anything more to an engine that the octane level that and engine needs. In other words, running higher octane in an engine doesn't give the engine any more or less performance unless the engine has high compression such as a built motor. The higher the octane, the more the fuel can be compressed before ignition. Lower octane tends to ignite sooner when under compression. In stock engines, lower octane gas runs very well, while higher octane in the same engine really doesn't do any more for performance. Some folks report that their bikes run a little smother with higher octane, I'm not sure if this is just weather related or I'm feeling good about putting high octane in my gas tank kind of feeling. As a pilot, I could swear the engine sounds totally different when I'm flying over long water stretches that when flying over regular ground.
As to your plug fouling, that could be caused for a number of reasons like compression level, valve settings, ignition timing and or carb setup. I know that covers the entire gambit but all things should be checked / and adjusted to eliminate the obvious.
Before you can adjust a carb for mid-range performance, you have to make sure that your plugs look ok for full throttle operations. Try putting in new plugs and do a chop throttle test, run the bike at full throttle and kill the engine and check the color of the plug. If the plug is normal color - then your main jet is sized correctly. Now you can move on to adjusting for mid-range by changing the pilot needle clip up or down for mid throttle. Usually, this will adjust the throttle response in the mid range area. I've have not seen where a plug gets fouled at mid-range, but I'm sure it does happen. Your air screw is used to set idle. After all that, your pretty much completed adjustments to your carb.
If your ignition timing is a little too retarded or too advanced, it makes sense your not burning the full fuel load in the cylinder. If it was too advanced, you'd more than likely hear it ping from pre-ignition. That doesn't sound like what you have going on. If the ignition is too retarded, then you might see what your describing.
Finally - compression, This really tells you the health of your engine. I'd check it. I'm a real believer in covering the basics first. I don't know how many times I've been caught chasing a problem when I over looked an obvious thing. Not knowing any more than what you've said here, I'd check these items in the following order: compression - ignition timing - valve settings and finally carb settings. You may very well have carb problems, but checking the other items on my list does not take long and can be indicators to the real problem.
Keep us posted on what you find.